Santa Unkillable

BASIC GUIDANCE

239-241 - Pain Keeper
205-210 - Stun Tank (Skullcrusher/Taurus)
188 - Cleanser/DPS
171-178 - DPS (faster than Sir Nicholas)
171-176 - Sir Nicholas

UNM
Skill orders:
Pain Keeper - A1, A3, A2, auto
Sir Nicholas - A2, A1, A3, auto
Skullcrusher - A2, auto
Cleanser - A1, cleanse

NM
Skill orders:
Pain Keeper - A1, A1, A3, A1, A2, auto
Sir Nicholas - A3, A2, A1, A3, auto
Skullcrusher - A2, auto
Cleanser - A1, A1, cleanse

Stun targeting is always an issue, so avoid Lifedrinker mastery on your stun target. Champions like Bad-el-Kazar can provide too much healing and should be carefully considered before using.
Using Rust's Stun Calculator can certainly help in setting it up, however it's only an approximation, not exact.
All the speeds listed are true speeds. When tuning for only a single difficulty, there is considerably more of a speed range available.