Obliterate X

Whenever this attacks, destroy the defender's X lowest tech units first.

Obliterate X Rulings

Obliterate never targets. Having resist or being untargetable doesn't help against obliterate.Sirlin, 03/14/16
If there are any units that are indestructible or that can't leave play, ignore them when looking for the lowest tech unit. If such a thing would be their "lowest tech unit" then instead destroy their next lowest.sharpo, 11/24/16