Kashhrek is really the best 4* tank, thanks to his large elemental defense against fire.
That makes stacking red against him precarious, and his healing helps make up for damage from a neutral stack that a smarter attacker will use.
If he’s not killed promptly by a good board, he can be quite troublesome.
Suggesting Caedmon as left wing on that team is meant to goad attackers into stacking red, either based on the tank color, or seeing two greens. A smart attacker won’t, and will stack green or yellow instead, but typically at that level of play a lot of players will be tricked and play right into Kashhrek’s strength by bringing multiple reds.
And in terms of left vs. right, as the AI fires specials from left to right, it ensures Caedmon’s dispel will fire before other hitters if they happen to be ready on the same turn. That’s useful in case the attacker happens to have a defense buff or Perfect Riposte active.
Tiburtus and Grimm as wings will have good likelihood of firing while an attacker futzes with Kashhrek, and putting Tiburtus left and Grimm right ensures that if both are ready to fire on the same turn, there’s more chance of Grimm hitting an already-debuffed target with his heavier hit and finishing them off.
For the right wing, I went with the only other fast sniper available, to increase likelihood of firing in a timely fashion from the corner.