I have all three fully leveled and partially with emblems.
Really depends on your line up. If you don’t have Grimm, or Gormek then possibly Tiburtus first.
Mana suppressors are good, but defense down hero is more versatile. For challengers anti mana heroes rule.
Maybe Merlin if you don’t have Proteus or any of the other Tier 4 mana stoppers/preventers/silencers. <Cabin Boy Peters, Hansel, Gretel, Proteus, or Tettukh>.
Frankly, I got Gretel before Merlin and pulled Proteus at the same time. Although Merlin can be useful, I just don’t use him much.
Cheshire cat has better Def down than Tiburtus, I was using him for titans, but if he fires before his DOT finishes, he is not adding a lot more damage. I don’t use the cat anywhere else.
I found it annoying to weaken the tank and then have him swap in a full health corner…lol