If you have a save before getting the battle tower you could always reload it to return to Broken Valley to get the achievement, though you would loose the experience from killing it going back to your recent save.
The killer bunny can not jump or climb very well, so if you are near a rock or platform, or he spawns at the bottom of a steep river bank you should be able to take him out from a distance, as Jdog619 said. Your creature or a summon may not do any damage, but would keep him distracted and not moving around, allowing you to hit him more consistently and for more damage.
