helmet, poison, vampitism and revive was my go to items, very useful for final boss
and 2 of each buff (health dmg and speed)
oh, and you can talk to the shopkeeper, just say happy things and happy things will happen.

normal enemies:
not sure how to explain, but when enemy does that attack animation, try to be on the other side from where he looks at you, not where he stands at the moment
*so if he goes left, but his body is at the right side, he will aim at the middle
*and if he aims left and looks left, he will aim left
and vice versa on the right side

first boss:
-when you get him into 2nd phase, best option is to stand in the middle, the bullets mostly go to the sides of the area, so just stand still and hit as fast as you can

second boss:
-if you're having a hard time tracking the bomb, hold the S button and you'll have a better view

final boss:
-he moves faster you more he's damaged, so i did 2-3 clicks and moved away.

good luck!

Well, there is a speedrun by a user named liladriel05
which he does it fast, just as a speedrun should be, great stuff




TLDR: just buy as much hats and wallets as you can.
And 1 health, 2 dmg, 1 speed.

good luck beating that record, I sure aint gonna try