Burns and negative effects are definitely the trick with Krampus, tho not all negative effects stick.

Things I've found helpful that stick to Krampus:
* Burns of all types
* Heal block
* Perfect corruption
* DEF down if physical attacks, MR down if magic attacks (tho mainly if they come along for the ride)

Things that definitely don't work on Krampus:
* Stun
* Exhaust
* Freeze
* Sleep