Obliterate X

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

Obliterate never targets. Having resist or being untargetable doesn't help against obliterate. — Sirlin
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