Quick answer
Start with role coverage
Early accounts have the clearest reported pair: Bone Guard in front and Skeleton Archer for ranged damage. Midgame guides point toward Mighty Orc plus Lava Imp; late and endgame targets shift to Hellfire Warlock or Demon Lord, Skull King, and Deathlock support.
The game’s Update 3 listing says battles and balance were improved, but it does not publish a full change table. Treat every recommendation below as current community guidance and check your owned units before spending rare upgrade materials.