Bunny Bagger is a secret achievement in Divinity II: Ego Draconis. All you need to do is kill 30 to 35 rabbits, then the Killer Bunny will make an appearance. Once it appears, the screen will zoom to its bloody face - immediately after, JUMP AWAY! It will spawn very close to your character an will ALWAYS hit in melee. There is no level requirements to fight it either, so it is possible to make it appear when you're at level 15, however if it lands a blow at that low of level, you will die almost instantly.
The Killer Bunny's preset level is 22, an if battling it at a lower level, use range or minions to take it down. The best tactic is to find a place off the ground (like a large boulder at least 5 feet in height, just to be safe) and have your creature fight it and shoot at it with your bow or if you're by the village in Broken Valley, lead it to the village to get some friendly's to help - just be warned that if they kill it, you won't get any experience.
There is however a monster exploit, but it could have been corrected in "The Dragon Knight Saga", but the exploit only works on melee only enemies - look for a river or even a lake that your character can stand in (without going into swim mode), an if you use ranged attacks on them, it will make the battles feel like a cheat (They'll ignore you completely if you are in water, even if you shoot them with arrows and/or magic, even your creatures will stay away from water), but if you go to the other side of the river or lake, they'll either teleport (not magically) or look for a way to get over to your position. For the Killer Bunny, standing in the water is not a good tactic to take it down, the Bunny has an anti-cheat ability that causes it to heal within seconds.