The Heat is On Achievement in Bright Memory: Infinite
The Heat is On30
Defeat 20 enemies using flame-type weapons or skills
How to unlock The Heat is OnShelia's shrapnel bombs – the secondary fire for her shotgun  – will set enemies alight, so this is a good bet for bagging this achievement. Just be certain that you defeat them using a flame round, or one of Shelia's fiery abilities. And if you're having trouble, use the Tractor Beam and EMP  to suspend enemies in stasis, and then pick 'em apart. Do this to twenty foes, and the achievement should unlock, although you'll need to go out of your way to do this – during a full playthrough, this didn't unlock for me, so make a beeline for flame kills whenever possible.