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Target creature gets +2/+0 and gains first strike until end of turn. (It deals combat damage before creatures without first strike.)
If a creature doesn’t have first strike, granting it first strike after combat damage has been dealt in the first-strike combat damage step won’t prevent it from dealing combat damage. It will still assign and deal its combat damage in the second combat damage step.
Thunder Strike can target any creature, even one that already has first strike or double strike. (If it affects such a creature, it will, in effect, just give it +2/+0.)
