There are some glitches/bugs which you can use.

Level 1.

• First enemy: you need to sit down (for one frame) and throw shuriken at the same time, so enemy's knife makes no damage.
• Next 5 enemies with knives: your shurikens must be seen on the screen at the same time with enemy's knives, you must walk forward (no jumps), and enemy's knife makes no damage.
• First 2 bullets after knives: you can jump over, but it's dangerous. Actually only first bullet can damage you if you crawl, second can't.
• Enemies with rifle: same strategy as for 5 enemies with knives. Level 2. I has not found too much here.
• Sitting enemies: there are two cargos with 2 sitting enemies with knives. Every first enemy u can jump over. (In game u can't jump over enemies).
• Boss: have no idea how this glitch works, but some time u can skip explosion animation (saves like 4 seconds). Level 3.
• Jump over water: have no idea how this glitch works, but u can jump over water like normaly platforms, it works right before the first enemy with knife. Level 4. Nothing yet.

Overall: only at Level 1 you can beat boss without POW, it's faster than to collect all bombs. Other 3 u need all bombs.