Arcane Horrors are powerful magic users that can warp around the battlefield. Take advantage of their low armor and light them on fire for the best results when fighting them. They are usually in the middle of a group, leading other Undead so focus on them first.
Corpses don't pose much of a threat by themselves but often come in large number. Be sure to defeat them at a distance before they get too close, Fire is your best bet since they can be inflicted with Burning.
Revenants have the ability to pull you in from range and then smash you with their massive shield and sword. Unlike the other Undead, the Revenant has a large health pool and very high armor so you'll need to use Fire to really burn them down but more than that, you require sustainability to outlast them.