Getting better at DOOM Eternal

Im late to the show, got a ps5 and only now i got doom eternal, having loads of fun since i dont know how long. My question is, because i do not want to lower the difficulty ( and i think i picked up third? ultra violence? ) and im getting wrecked at the first optional thingy where if "you solve it" youll get three weapon upgrade points. Forgot the name, purple thing you need to find a key first to open.

EVERYONE IS SO DAMN FAST! On which enemies should i concentrate, which ones should i leave for glory kills and ammo?

Artwork by Edward Delandre