This trick isn't so necessary on Level 1 or 2 but can be really useful if you, not a pro player of just going to kill yourself on Level 3 difficult (No upgrades, only basic pistol). Also this guide can contain spoilers for the game. Just saying.
First of all you need to kill the Boss.
After that, for example, you have low HP and worried about health kits. Everything that you need to do now is jump off the edge. Yes, i mean die.
(Ok, dead). Now you'll respawn, boss is still dead. But you not bleeding.
And Make Sure - Full HP
The trick is that game downloaded Drfiter status from the moment you entered the boss's room before he was killed. So you can gain not full HP and your ammo will recover too (or roll back).
Where it Didn't Work
It woks only on first 3 bosses of East, North and West. This trick don't work with boss and mini-bosses of South (Exactly I don't now).