Is there anyway that i can make death coil target enemie's undead units, i've tried changing the target allowed and the race of the unit but it doesn't work
no, you will have to change another skill to look like death coil, or change the clasifaction of the unit its self if that that dosen't mess your map up.
Sadly a lot of Animation and Target Classification thingies are hidden deep in the code of WK3. If you want the Death Coil to dmg enemy undead units, then modify all undead units' classification not to be undead. If this is an RPG map then do this with the undead units you want to be the target of the spell.
If you only want to make a damage dealing spell then use another spell. If you want it to retain the healing effect, and still damage enemy undead units of the same type of urs well.. thats' impossible.