Like JNP said, it sounds like you're gonna have to use proximity to emulate a hit between the projectile and the gas cloud.
Since there is no direct way to measure the distance between two helpers, go to your gas cloud and projectile helpers and add ParentVarSet state controllers (different variable from each helper) that will constantly indicate their positions in X (this is assuming you're not using projectiles that can be shot down at an angle, that could get trickier since you'd need two more variables for pos y). The character will also have to have a variable that indicates the distance itself. Once that variable reaches a value that would represent a projectile being at hitting distance from the cloud, then you go to your projectile and make it changestate to its "death" animation. Add a playsnd for the hit and an explod for the hitspark, if any.
I strongly recommend you register the result as an absolute. so from the projectile, it'd be more or less like so:
(This is assuming you're using var(15) for the cloud position, var(16) for the projectile position and var(17) for the distance)
[State 1000, ChangeState]
type = ChangeState
trigger1 = parent,var(17) <= abs(parent, var(15) - parent, var(16))
value = 1001
Hope we helped!
P.S: Mods, sorry for the double editing, forgot the damn code tags