My advice would be to make sure you come into the level with at least one of every powerup with you. Watch the Domineeress' health, if it's running low, then take out the horde surrounding it. If things get drastic, use the time stop powerup. Also, Dagon's End is NOT the final level.
Ỉ̸̡̙̞̆̑t̴͈̘̑ ̸͇͗̔o̴̲͊n̴͓̒l̴͓̚y̴͍̽̅͗͜ ̷̬̋g̵̩̮̀́̈́ȩ̵͗ť̷̘s̴͇̎̉ ̵͔̗̚w̵̼̓̄͘o̷̖̘̣͂̈́̕ṛ̵͕̜͑s̴̛͔͇̜ë̷̯̬̮́̊ ̸̺̏͜f̴̨̘̐͜͠r̷̺̝̘͠ȏ̵͕̦͎͐͊m̵̖͕̎̎͜͠ ̴̨̗̪̓h̶͕͇̀̌̀ͅe̵͕̭̽ṟ̶̡̇͐̈́ĕ̷̢̞̺͊ ̶̬͂ö̴̼̯́n̷̡͉̳̍ ̵͖̔͑̔ȍ̵͓̞̞u̸͍͔̼̿̐̄ṯ̷̍̔̾,̶͇͕̺͝ ̷̯͋́f̷͎̬̦̿r̴̡̝̿͑͝i̵̖̤̐é̷̞̝̞n̵̥̓̚d̵̥̱͈͝.̴̰͐̀