Ikari_warriors_2

Level-by-level description

Level 1

https://www.youtube.com/watch?v=MqlbCMI5a_g
The boomerangs have mainly three advantages:

1. It eliminates the risk of getting hit in the first section
2. Less risk of getting hit by the flying enemies, but not entirely eliminating the risk
3. Takes out the pursuing enemy instead of tanking a hit (for a cost of 2 HP)

Level 2

https://www.youtube.com/watch?v=06dNHIjtdgI
An easy-to-execute and very consistent way to get through the level. Collecting the life power is often useful since most attempts will include one or a few Galago tunnels with enemies.
The fastest speedrun strat is to use the thunder attack on the boss. However, missing the thunder attack leads to a slow boss fight. So using the arrow instead is therefore a good compromise in a marathon setting.

Level 3

The speedrun route is mostly consistent already. You'll occasionally take a bullet hit, which could be prevented by changing to the boomerangs though.

Level 4

https://www.youtube.com/watch?v=cpLriRqL6io
It's suggested here to use the boomerangs, since that eliminates some of the risk from taking a bullet hit. Often, you'll anyways need to use the life power at this point, so a menu trip is likely to anyways be needed.
The life power item at the end requires a small detour, but is still recommended since the tunnel at the end and the level 5 both often result in additional damage.
Since the arrow was used on the second boss in this route, it's here suggested to use a combination of the earthquake power and the bazooka. The same combination could be used on the second boss, but is less suited since the boss's claws are unaffected by the earthquake power.

Level 5

Assuming the health is not too low, the speedrun route is fairly straight-forward to execute. There is a significant risk of taking bullet damage at several points though. If health is low, the boomerangs could therefore be used to mitigate some of that risk.