The Rat Ogre is a huge rat that spawns and acts like a tank from Left 4 Dead. This achievement means you have to do the last hit to kill it. It will not count if you do the most damage, you just have to do the last damage. In the bottom left of your screen it will say:
[insert gamertag] killed Rat Ogre.
Some tips to kill the rat ogre are:
1) Use projectiles
2) Dodge backwards with
+
if he is chasing you and he will rarely hit you
3) After a good amount of damage has been done use a bomb if you have one
4) Melee him in the back if he is focused on another enemy
And that's it! It can be done on any difficulty.
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Happy Hunting