Both Doom and Abomination only have one Heroic Feat, to defeat 50 enemies with their heroic attack. Go into Challenge mode, select either Doom or Abomination to start with and pick whatever survival challenge you like. I used the first one with the Skrulls for Abomination and Asteroid Elevator for Doom (see "Fear the might of Doom?" for why). Basically fill your meter, get in a group of enemies and pull to unleash your heroic attack. Repeat as needed. Once you have the first character's achievement, back out to the character select screen with , select the next person and start over again.