"ncinerate is a tech power available to engineers and infiltrators. Unless Shepard is one of those classes, the only character with this power is Mordin. Armor is denoted by a yellow bar. You have to deplete the Armor bar completely for it to count. Your companions wiping out a shield in one hit will count for the trophy, but you have to trigger it using [R2 Button] . Unless you’re playing on Casual or have any evolved form of Incinerate, knock the enemy’s Armor down about halfway, and then fire Incinerate.
You will encounter plenty of armored enemies throughout the game. Mordin’s loyalty mission and any mission involving the collectors have a high number of armored enemies.
When you encounter a group, incinerate their armor and then reload the checkpoint. Your progress will save."
Sources : http://www.ps3trophies.or.org/game/mass-effect-2/guide/