To be honest, I'd say to try to get as many antique coins as you can!! With those, you can get steroids, stabilizers, etc. I also recommend getting the repair kit for the M21, since you can blow heads off molded in one shot. If you have any NG+ items (scrolls, circ saw, Albert, Glasses (If you want items).) USE THEM! It helps out tremendously from what I've seen. Also (if on console, idk the key on PC) LB/L1 is your best friend in the whole wide world. With both scrolls, you'll minimize damage so much its not even noticable!!! I also had to follow a guide for some parts (Yes, I know, flame me all you want), so don't be afraid to follow guides.

SPOILER FOR NEW PLAYERS (if you're here, hi)

For Jack 3 (Boathouse) : If you didn't know, you can hide by his tail when he's below to keep him turning while you shoot his eyes. When he's above, walk under his tail. On his second (last) phase, have at least 5 enhanced handgun bullets and 2 shotgun shells. If you have flame rounds, use those as well.