The basic idea behind a dummy caster is:
1) You make a unit with no model and the Locust ability.
2) You make a trigger that runs when your shockwave ability is cast.
3) You create 1 dummy caster at the position of casting unit.
4) You give it the ability you want it to cast (in this case, a 0 damage shockwave) in the trigger.
5) You order it to cast the ability.
6) You give it a short expiration timer (~2 seconds should do)
There are also some fiddly bits like point leaks, but if you're just starting triggering then you shouldn't worry too much about that.