Trouble with the 'Doom' ability

[Lashkor]

Ok, so I based an ability of mine off of Doom, a single target silence/dot. Thing is, the ability has 4 levels to it, but after the first level all creeps become too powerful. I'm not really worried about it, as I can't see why people would want to cast the spell on creeps anyway, but is there anyway to fix this?

 

[Dr Super Good]

You can reduce the level of creeps in their unit data. You can also change the max allowed level if I remember correctly, check for a field in the ability data or else check in the game play constants for a field that affects dooms max allowed target level.