Metroid Fusion is notorious for its difficult boss battles, and as such, strategies are provided for all bosses.

Arachnus

Any%

You should be coming into this fight with 20 missiles. Simply stand in front of him and spam missiles. He takes 15 missiles to go down, and his core-x takes 3 missiles, requiring 18 missiles total.

Arachnus has a random chance to roll, which he is invincible during this animation. If you are spamming missiles, you will usually tink 2 missiles on him; however, this is fine as you can afford to waste two missiles without having to rely on the core-x's drops.

Always leave at least one missile for the core-x (two if you want to double missile it at the start). If Arachnus is not down when you're at one or two missiles, then use your power beam to finish him. This should generally only happen if he rolls at least twice, which is very bad luck.

100%

This is more difficult than any%, as you come into the fight with only 19 missiles. If Arachnus rolls even once, you generally won't have enough to finish off the core-x without getting drops. So try to predict if he'll roll or not and pause in your missile spam to minimize your chances of tinking missiles when he rolls.

If you think more than one missile, you must get drops from the core-x.

Charge Beam Core-X

This fight is the same in all three categories.

Stand on the edge of the lower platform, where Samus is standing on it but still able to shoot missiles down. The core-x will be stuck below the platform, aiming diagonally, so it cannot hit Samus. Shoot missiles down when its eye opens; it takes 4 missiles to finish it.

You can finish it in two rounds rather than four if you can manage to do two double missiles, as this is the only beam core-x where it is viable to double missile it outside of TAS. Jump into the ceiling above, and just after you start falling, fire your first missile, then fire the second missile as late as possible before landing. The timing is very precise, but as long as you have enough missiles, there is no reason to not go for it.

Zazabi

Zazabi is a very random boss. He should typically be taken out in 4 rounds.

It is important to note that once you do enough damage in one round to destroy a segment, any extra damage before the next round does not add on damage.

When Zazabi is jumping around, pay attention to how he's jumping. If he suddenly jumps at a slower horizontal speed, then he's about to open his mouth. Get in position and fire your charge shot upward, followed by a few missiles. The number of jumps he does in the first three rounds is completely random; in the last segment, he will always open his mouth.

For the first three rounds, one well-aimed charge shot is enough to end a round. However, it is very easy to be slightly off on the aim and have only half the charge shot hit. So it is advisable to fire an extra missile to ensure that you finish the round, in case the entire charge beam doesn't hit.

The fourth round requires a charge beam and two missiles to finish him. It is recommended to fire three missiles instead of two though, for the same reasons as above.

Deal with the Core-X as usual. It takes four missiles to finish it.

Serris

This fight is the same in all three categories.

Serris has 50 health, so he can taken out with two close-range charge shots (a full hit with the flare does 26 damage). It is important to finish him in as few shots as possible, as every time he takes damage, he starts speed boosting, and he is invincible while speed boosting.

Serris has several patterns he can use:

• Straight across the room, left to right
• Straight across the room, right to left
• Zigzagging the platforms left to right, then jumping out right to left
• Zigzagging the platforms right to left, the jumping out left to right
• Serris dives out of a random gap and dives into another gap two gaps away

When the fight starts, Serris will always do the fourth pattern. You can consistently hit a full charge by either:

1. Climb on the very left edge of the ceiling runs and aim left. Serris will fly straight into you; you can hit him here.
2. Stand on the right edge of the 2nd platform from the right (the platform you step on when the floor crumbles) and aim diagonally up-right. Serris will come down directly in front of you.

The first option is generally the most preferred, and is faster as it hits Serris earlier. You should always be on one of the two center platforms, or on the rungs above them.

Dodging Serris is very easy once you know his patterns, and you want to dodge his speed boosting attacks since he's invincible during this time. Holding the ceiling runs will dodge all of his patterns except the latter half of both zig-zag patterns. If he uses a zig-zag pattern, then drop down after he zig-zags, then grab the rungs after he jumps over you.

For the second charge beam shot, the strategy comes down to reacting to which pattern Serris does and hitting him up-close. Pause buffering is a good starting strat, especially if you are focusing only on in-game time.

• The easiest pattern is the first two; you can easily just shoot him from standing on the platform just before he hits you.
• If he zig-zags right to left, you can shoot him on the 2nd platform from the left, or shoot him on the rungs above the 2nd platform from the right
• Vice-versa if he zig-zags left to right
• If he uses the last pattern, with a single small dive, and he ends his dive near you, you can hit him with a diagonal shot. However, his hitbox will be farther than if he was diving from a zigzag, so it is very difficult to hit him with a flared charge shot; it's advisable to simply wait until a different pattern.

BOX

This fight is functionally the same in all three categories. However, you want to make sure to miss as few missiles as possible in any% and especially moreso in 1%, as you need every single super missile in 1%; if you use more than 10 super missiles in 1%, you will likely lose time later on as you need all 5 remaining super missiles.

BOX takes 10 super missiles to defeat. Run up to BOX aiming diagonally down while it's stuck in the wall. Get close to it, jump and shoot a charged beam and a missile diagonally down. If BOX is hit at the very start, he will immediately start jumping, which is what you want. The charge beam's purpose is not to deal damage (it doesn't affect how many missiles it takes to finish BOX), but rather, as insurance to make sure you can hit BOX to make him jump in case the missile misses.

After the initial hit, immediately grab the right edge of the rungs above. If you're fast, you can squeeze in some super missiles by shooting diagonally down-right before BOX is directly below you. When he is below you, you have just enough time to pump 4 super missiles directly down before he opens his cannon.

When he opens his cannon, climb to the left above the bomb. When he puts away his cannon, you can fire more supers diagonally down-right before he jumps again. Ideally he should be dead at this point.

If he is not yet done, then climb to the right edge of the rungs again (the fire pillars won't hit you) then wait for him to jump below you. Fire missiles directly down.

If he's still not dead, then just repeat the above two paragraphs.

Wide Beam Core-X

Yakuza

The general strategy for Yakuza is the same for all three categories; minor difference will be mentioned. This is the most complicated, trick-filled, and fast-paced fight in the game, making it likely the most difficult boss to speedrun overall.

Phase 1

The moment you land and the ground shakes, you can damage Yakuza before he comes down because his mouth is open to shoot fire. Stand in the center of the room and jump up, firing ice missiles while jumping up repeatedly. You'll hear him screech if you're damaging him; the missiles will only hit if you're jumping up.

After that, Yakuza will crawl around the room between 1-4 times randomly. To skip part of him crawling around the room once, you can jump into his claws to get grabbed, then break out by mashing left and right; this saves a little bit of time as long as you get grabbed higher up. Obviously you shouldn't do this in 1%.

To dodge him crawling, simply morph in the corner. This usually works; it only does not work if he starts crawling from the very opposite corner of you (i.e. if he stops at the very top left corner and starts crawling, he will grab you if you're morphed in the bottom-right corner).

Whether you get grabbed intentionally or dodge his crawl, whenever he eventually opens his mouth, fire one charge beam then a volley of missiles. As long as you damaged Yakuza before the fight started, you should easily be able to finish this round the first time he opens his mouth.

Yakuza has 1000 health. Once you have dealt at least 500 damage to him, he will start round 2 and reset to 500 HP. Thus, doing extra damage does not help unless you can somehow bring him to 0 health in one round.

Round 2

Have a beam charged while Yakuza is transitioning. Just when his legs break off, you have just enough time to squeeze in a charged wide beam shot along with two ice missiles to quickly take off 125 damage, before he starts jumping.

There are two strategies you can use here: missile strat or charge beam strat. The missile strat is faster than the charge beam strat, but much more difficult and requires a strong instinct of knowing how yakuza moves randomly. In 1%, the charge beam strat must be used as you do not have enough missiles for the missile strategy.

Missile Strategy

The faster and more difficult option. The idea is to hit Yakuza with a missile as quickly as possible after each jump by jumping up with him and aiming properly. However, you have almost no time to predict where he'll go, so you need to have a very strong instinct of this round in order to use this strat. After hitting him with a missile in the air, land, then immediately jump to where you think he'll be next, and shoot a missile into him again. Repeat.

If you find the charge beam strategy too difficult to time, then feel free to fire missiles from the ground instead. While this is significantly slower, it gives you much more time to know where he'll be, and more room for error.

Charge Beam Strategy

For this strategy, Samus will stay on the ground the entire time, using charged wide beam shots. Each complete space jump takes just enough time for Samus to barely charge her wide beam. So after Yakuza finishes a space jump, hit him with a charged wide beam shot then immediately start charging again and repeat.

If you miss a shot, then it is recommended to skip Yakuza's next space jump to ensure you have a fully charged shot. It is very easy to release the shot just before it's fully charged.

You must use this strategy for 1% as you do not have enough missiles to use the Missile Strategy.

Regardless of how you defeat Yakuza's second form, defeat his core-x as usual. It takes 6 ice missiles to defeat his core-x.

Nettori

Nettori has 2000 HP, and is one of the longest bosses in the game. Before entering the arena, immediately drop a power bomb just before falling down the crumble block, as it clears the pollen early in the fight to let you get on the safe spot, and also damages Nettori.

There are two safe spots that Samus can stand on reliably safely without getting hit by the pollen.

1. Samus cannot be hit by either flower if she is crouched on the very left edge of the right platform.

2. The left flower will stop dropping pollen if Samus is standing on the very righ edge of the right platform; the right flower will not hit Samus.

It is recommended to stand on the right edge, as it brings Samus closer to Nettori and allows her to damage the boss more quickly, but it is somewhat negligible and comes down to personal preference.

Strategies on which weapon to use depends on the category, as this is the one fight where having a high missile count changes how to damage Nettori.

Regardless of which strategy you use, after phase 1, Nettori starts firing plasma shots. If it fires a high shot, crouch to dodge it. If it fires a low shot, do a short hop and shoot at Nettori high. Regardless of which height it shoots at, you can always hit it with an ice missile at standing position.

100%

Samus's best form of DPS is spamming ice missiles, at least humanly possible. Planting power bombs will only slow down shooting ice missiles, and does only 10 more damage, so it is not worth planting power bombs (except dropping one for starting the fight). Charge beam should not be used either, except as the very first shot when starting the fight.

1%

Be prepared for a very tediously long charge-beam spam fest, as you only have 15 missiles and 10 power bombs.

Use a charge shot, then immediately after shooting it, fire a missile then start charging again. Repeat this until you're out of missiles. Whenever there isn't a power bomb on the screen, plant a power bomb in between charge shots as well. It is important to prioritize not taking a hit nor fall into the flowers, as the plasma beam does a lot of damage to Samus, and two hits will kill you.

Any%

This fight can vary depending on how many missiles you have, and is the most complicated strategy; it is basically the 1% and 100% strats put together. 50 ice missiles are exactly enough to finish Nettori, so adding on the initial power bomb and wave beam shot, you can finish Nettori in about 48 missiles. In the ideal fusion any% route, you'll only have 40 missiles, but if you collected more expansions and have at least 50 missiles, then feel free to use the 100% strat.

Otherwise you'll have to compensate for the missing missiles. Take 48 - the number of missiles you have, and that's roughly how many charge beams and power bombs combined you will need to compensate. It is advised to fire an extra charge shot or two, as it is very easy to tink a missile on nettor's plasma shot. So follow the 1% strat until you've compensated for those missiles, then start spamming ice missiles.

Nightmare

BOX-2

Ridley

SA-X

Omega Metroid

The speedrunning strategy is the same for all three categories.

After watching the SA-X get swiped by the Omega Metroid, charge your beam while jumping into the ice beam ability. Your beam will charge during the acquisition animation, which will let your first shot deal more damage.

The key to defeating Omega Metroid is to deal damage to it so fast that it continuously staggers. While it is against the right wall, there is a specific spot that Samus can stand such that by shooting diagonally into Omega Metroid's body, the center beam of the ice beam travels through it, dealing damage for every frame it is inside it.

Run towards the Omega Metroid to lure it to swipe you, dodge it by running left, then run into position during its cooldown from swiping. Once you're in position, start shooting the ice beam as fast as possible, and if you are shooting it quickly enough, it'll continuously stagger until it dies. If you feel like you're not in position, lure it to swipe again. At all costs, do not let Omega Metroid walk left or else you will lose your position, and you will have resort to the slower strategy of repeatedly running in, shooting a few times, then running out.