To gain this trophy, you must either choose Engineer or Infiltrator as a player class, or use Mordin if a different class. The Incinerate ability is an unlocked power as the first two classes, and available by Mordin right away.
You need to Incinerate the armor (yellow bar) of enemies 25 times. Please note, this is not your incendiary ammo, only the Incinerate ability will unlock this. You must also completely remove the armor for it to count.
A good mission to do this on is Mordin's loyalty mission. You are required to have him in your party, so even if you are a class with this ability, there are a lot of enemies with armor.
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