When a resolving effect modifies the characteristics of objects on the battlefield (such as power/toughness), the permanents that will be affected are “locked in” as the spell/ability resolves. So when you cast Mutilate, it applies -X/-X to all creatures currently on the battlefield as the spell resolves, and that effect will last until the end of this turn for those specific creatures.

For example, your opponent controls a Doomed Traveler and a Strangleroot Geist (with no counters), and you cast Mutilate with 4 swamps. Those creatures each get -4/-4 until end of turn. The Traveler’s spirit token will enter as a healthy 1/1, because it wasn’t around to be affected by Mutilate, and the Geist will enter as a healthy 3/2 for the same reason.

This is different from Elesh Norn, Grand Cenobite, whose static ability will apply as long as she’s on the battlefield, and will affect all creatures on the battlefield no matter when they entered.

Today’s Rules Tip written by
Josh Stansfield, L2 judge from Orange, CA