This is one of the glitched achievements in the game, so you'll need to grind more than 50 kills with them.
Heroic abilities are activated with LB and RB
Ultimate Abilitie is activated with LB+RB
You'll need hero orbes to recharge them (yellow orbs that the enemies drops)
Just use your abilities once the enemie has low health, you need to get the killing blow with the ability for it to count, as it varies from character to character just use the one you are most confortable with, i got in a couple of hours with Black Widow's ultimate ability on lower difficulties.