...and if that doesn't work too, base your skill on another similar ability (instead of blizzard, you can use warstomp repeteadly) And If you ask "why would I want to do that?" answer is simple, object editor fields (especially targets allowed section) is sometimes may not work flawlessly. Some skills does not affect destructibles no matter what you do, or the caster itself.
Anyway. If you want to damage certain units (where object editor doesn't help) you can trigger it with Unit Group actions using Matching Conditions . This way you can express which things you want to take damage with exact settings.