Assault Terminators are a kick-ass Space Marine unit. Very tough, very big, and very strong. Difference between normal Terminators and these is that they are fighting in melee.
Look, these guys can destroy everything in the game. They have Thunder Hammers, which makes heavy melee damage (Force Commander's Thunder Hammer makes power melee). So, you can teleport them back of a tank, and start hitting from the rear. If there is a walker, you can just stop it from attacking your other units. They also have Storm Shields which gives more health and makes them even more "tankier."
Their only upgrade is Lightning Claws. Let's look at the bad side, you are losing heavy melee weapons and getting power melee weapons. You are losing shields and losing health also. On the other hand, you are getting the best power-melee weapon in the game! You can destroy every kind of infantry with these. They are using full-upgraded Nobz? Get these and see what happens. Do you know what will happen? They will eat the Nobz in seconds!
Still, because of they are fighting in melee, they will take always more damage than the normal Terminators so for their survival, don't use their teleport ability to get into combat. Save the ability for escape since it has a very long cooldown time. They are slow and this makes getting into combat with the rest of the army hard. You should use Veil of Time as a solution.