If a spell targets multiple things, you can't target it with Imp's Mischief, even if all but one of those targets have become illegal.
Imp's Mischief
You don't choose the new target for the spell until Imp's Mischief resolves. You must change the target if possible. However, you can't change the target to an illegal target. If there are no legal targets to choose from, the target isn't changed. It doesn't matter if the original target has somehow become illegal itself.
You can't make a spell which is on the stack target itself.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Thomas M. Baxa
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