If you just wan't to kill the Killer Rabbit for the achievement, then this is how I killed it the easy way:
1. Have multiple characters.
2. Have a character in high level running around the fjords.
3. Have a character in low level do the white rabbit quest by Sosostra.
4. Save and kill rabbits in the area around Sosostra until the killer rabbit jumps you. The Killer Rabbit is loose!
5. Load the high level character and kill some rabbits - The Killer Rabbit will appear!
6. Easily chop it's head off.
