Learn which yellow dudes to farm (they give you a tonne of health). If you need to take a hit, get hit by the yellow dudes rather than their bullet things. In the big room with the key thing I also have a pretty safe strat where I drill a pocket that I can stand in and not get hit while I blast the yellow dudes with Kilver .

P.S. I don't know what anything is called in this game because I'm pretty new still.