TargetPoint is where you click on the ground when using the ability. Or in other words, the point you wish to have your missile move to. TriggerUnit is basically you, the caster of the spell.
Rhymer's way can work if you set the missile speed higher. Just gotta test it till you get the speed and the timing of the stun right. But yeah, DiFm's triggers would be better and more recommended. It's good to know how to make slide triggers, anyway.
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.