This card is affected completely normally by spell cards.
When targeted by a creature or permanent ability, first the ability takes effect, and then the skill is gained.
Ex. Eternity uses RT on mirror; mirror is RTed
Ex. Fire bolt is used on mirror; mirror loses appropriate HPs, no other effect
Ex. Black nymph uses LS on mirror; it gains the vampire skill but is not poisoned (does not gain LS skill because it is replaced by vampire)
Ex. Earth nymph uses BB on mirror; it gains 20HP, is not delayed, and gains the BB skill, costing 3
Ex. You cast Pandemonium. Mirror is hit by a random effect. It takes damage normally but if for example the effect is congeal the mirror will be unaffected.
I hope that clears some things up.