how will you block them? with a crystal of secrets? mages have a counterspell.

Spend a turn on a counterspell so that the enemy doesn't block the titans with a crystal of secrets? And you can also block not with something, but with someone. Let the titans not have a penalty in melee combat, but they will still take a beating.
But the main thing is not this, but the fact that, due to their "bulkiness," the titans don't really need ranged attacks, and if they could be made even stronger, but ranged attacks were removed, then this would give mages more advantages.