U mean u want a spell to slow a unit not to stun? Use acid bomb. But if u want the stun and the slow. Make a dummy with acid bomb and order it to cast on the target
Use Acid Bomb and set the AoE to 1, it's still single target and will only affect units within 1 range of the target, aka standing directly on top of the acid bombed unit.
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.