It's not that hard, all you need to do is to replace that "base ability" (carrion swarm) in object editor with shock wave (modified so that it deals no damage, that distance is only 1 and aoe is also 1) as you do that give this ability to your hero. Then go to trigger editor search for the trigger of fire wall spell and change condition to ability being cast equal to shock wave(the spell you just finished in object editor)
The rest should function, without any problems.