Finally Haste get some buff, why can't Sez get a chance? I have some suggestions:

Option 1: Sez's skill 3 is very strong, but if it can not kill the enemy, Sez will become useless, so I hope that adding some unique effects on skill 3 will help him more useful.

Skill 3: Conviction 

Add: If cannot kill the enemy, inflict "Imprison" (can't be missed or resisted, can't be dispelled) last until the caster get a turn. When Imprison end, activate Enroach.

Imprison: the hero with Imprison can not attack or be attacked, can't receive any buff, debuff, healling and combat readiness. The effect of artifact and passive is negate.

Option 2: I hope Sez will be more OP if he can sustain good or revive, but we have too many revive unit. So I think "Shadow clone" is a good option.

Skill 2: Enroach 

When Sez die, Summon a "Shadow clone", last for 1 turn, to replace him.

Shadow clone can not be attacked or receive buff and have all stats equal to 50% Sez' stat. When allies is attacked, The Shadow Clone reduce 20% damage received and counter with Enroach.