[Trigger] AOE Polymorph, at end of duration

[Sawwy]

Hello guys. I've been trying to create a spell which is based on polymorph. But I want it to be "spread" to another enemy within a certain aoe (300) around the target at the end of the duration.
But everything I try fails miserably. Probably because I've been trying it wrong.

Any ideas on how I should do that?

 

[Tasyen]

Add SpellTarget to a Poly-group.
Loop 3 to 10 times per second over the Poly-group.
If an unit inside the Poly-group does not have the poly buff:

Pick units around it, filter non valid targets.
let a dummy caster cast poly onto one of them.​

-> Results into 2 triggers.