Certain weapons cause Plasma damage. You can also add mods that will make a weapon cause Plasma damage.
You can tell if the target died from plasma damage because they will slowly burn up and turn into a pile of ash (which you can still loot!)
Do this 100 times and the achievement is yours.
Edit: Thanks to Maxumilli0n for mentioning this needs to be done on one save file. If you score some kills, but do not save, they will not apply.