Kelthur is pretty squishy, as in he dies easily. He has really good damage but pays the price with being easy to kill in stages just like Baden. This sort of thing can make a hero useless for pushing difficult stages. There’s a reason why Brutus is so popular even though he is an otherwise mediocre hero, it is his survivability.
But there’s something you can do to improve Kelthur’s chances of survival so he can do some damage. Pair him with Tasi, and observe which hero Tasi freezes at the start. Then place him opposite of that hero so he swaps places with them at the start Right before they get frozen. Because of this, chances are no one will end up targeting Kelthur for some time which will let him survive for longer. This works especially well for infiltrating the enemy backline. Try it!