The lead champ typically is a good way to start.... he's a fan of stunning a DPS lead champ without the Steadfast mastery.

But overall, look for these things, whether Void or Affinity:

1. Does the champion have low HP/can he one-shot your champion?
2. Are they negative affinity?
3. Does the champion have these buffs on them – Increase Defence, Block Damage, Counterattack, or the Steadfast mastery (the Demon lord considers this as a buff, and it avoids it)
4. Do they have the lowest percentage max HP remaining of all your team?
5. Are they in the leader position? 

Side note: He does not consider Unkillable or Block Debuffs as part of his criteria