A dummy casting negative curse wouldn't work; it'd apply to all their attacks no matter who they attacked.
It sounds to me like applying a 0% Evasion buff on the person would be the best way, but I don't know if that will simply stack with any current evasion they may have or override it.
I can't get to World Edit, but it's something like:
Event: Unit is created
Condition: Unit type equal to (whatever your summon is; if you have different kinds of creatures, try classifying them all with a descriptor that no other unit in your game has, and change the condition to reflect that...
WC3 IE2 broke Warcraft 3 when I replaced the Storm.dll and ijl15.dll files. WC3, World Editor, and IE2 are unable to open (though IE2 was never able to open). I followed the link above.