You've whupped those Hellions but good.
How to Get Edit
Defeat 50 Hellions Damned bosses.
Hero side, the easiest way to get this badge is to hunt Damned in the Hell's Highway area of Perez Park, along the east section of the map. It is advisable to do this at a higher level, where it's possible to just run from group to group of Hellions, killing Damned and ignoring the others.
For villians looking for Damned, your best bet is head to Ouroboros. Beforehand, visit the Fateweaver and set mission difficulty so that you're equal to 4 villains. Reduce enemy level if neccesary. At the pillar of Ice and Flame select the story arc "Give em Hellion!" from the level 1-15 menu givin by Billie Heck. This is a 3 mission arc that will spawn many Damned in each mission at this difficulty.