The achievement is granted for winning a ranked multiplayer match using only "Tzeench-themed units". To clarify, there are units which are always Khorne, Tzeench or Nurgle, and units which are "neutral" but may get upgrades associated with one of the Chaos Gods. For Tzeench, only the Chaos Sorcerer and Heretics are always safe. You can also use Tactical Marines, Havocs, Dreadnoughts and Predators as long as you do not choose Khorne or Nurgle upgrades. The rest is out.