make it in a way the you pick units in a line and then change flying height up, then down.
try doing something like this:
advice, the way below is not MUI:
when the spell is cast you set 2 points (position of triggering unit + target point of ability being cast)
you turn on a periodic trigger that pick units in X range of (triggering unit_pos + any real (in algle of unit_pos to cast_pos).
picked units are addad to a group and are lifted.
when the trigger ends all units in the group takes damage.
this is easyer if you make not MUI, for sure, but you may use what you want.
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.