How would I go about doing this, i want it like a laser tag map where you attack ground and you shoot and it travels until it hits the target or dissipates etc
You'll want the missile to be a dummy unit which you can move with triggers and detect when the unit comes in range of a suitable target, and then trigger the effect.
There's several ways to go about this, the best and most smooth are vJass systems. In GUI, one of the best methods I found was Accname's idea of giving the dummy unit Permanent Immolation and using a damage event to detect when the missile would strike an enemy unit, saving you the lag of checking for units in range with a periodic event.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.