I got such an OP combo were when I get a zombie to spawn when it dies it drops 24 critters and when enemys die they spawn a zombies so I beat the boss in 3 seconds parasite is funni
you should try an explosion build, explosive auto shooting bullets, combined with upped explosion damage, and explosion during and after block. Combine this with 250% chance to create a new explosion after a explosion and i killed the final boss before its first attack. (if you want, you can make a semi explosion and summon build, it is so fucking fun