Everything should have a counter-tactic but I'm not seeing one for this yet.
Mages with maxed elemental damage and maxed initiative.
Tough melee units with high spell resist to protect the mages in case he lives long enough to charge in.
He WILL cast Despair, which is a "win button." You need to interrupt it any way possible. Pull Of The Earth or any spell that causes knockdown will interrupt a spell. Interruption spells will also work, but you have to pass his spell resist check. Bring multiple heroes with good spell mastery that can cast such spells.
Put entire army on mounts to avoid being knocked down by Titan's Breath.
Regular melee attacks trigger his melee defense spell that stuns attackers. Abilities like shield bash, cleave, crushing blow, etc do not count as a melee attack for those purposes.