Make Possesion spell target a specific type of unit

[Boris_Spider]

I'm trying to get Possesion to target a slave [Peasant]. All units are undead, and it seems I'm trying to fight the ability's hardcode targets allowed.

I've tried using:
Unit:
Combat - Targeted as
Stats - Unit classification

Ability:
Stats - Targets allowed

-Ground includes all units, I only want to allow the slave
-Air worked initially, but I now have an air unit capable of only attacking other air, which the slave is not, and the melee attack looks rediculous on the slave.
-Suicidal, Ancient, Vulnerable, Invulnerable, Item, Mechanical, Organic, Bridge, and Tree don't work. ~None of these would let me target the slave.
-Ward prevents use of the slave's build abilities, not good.

Is there a way to do this using Object Editor, or does it have to be triggered?
If it must be triggered, what's the best way? (JASS or GUI is fine, but note that I can't write JASS)

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Edit: Before anyone brings wonders, I've spent about a week looking through threads to do this, and found none that helped.

 

[Pharaoh_]

It works for me; cast it on the Acolyte and it will normally perform the spell, but, having Ghoul as a target, will bring up an error.

 

[Boris_Spider]

I figured it out.

When I set up the unit and ability, I did so using only 1 classification at a time.

I.E. Targets allowed: Ancient
Targeted as: Ancient/Classification: Ancient

as opposed to
~Targets allowed: Ancient, Ground, Organic
~Targeted as: Ground/Classification: Ancient

Thanks, I got it working again.

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