Multi-target abilities are not affected.
If you target your opponents creature then you'd have to target it with your offensive spells. You're still allowed to target your own creatures though.
Any feedback on the wording of the actual card? I went with "prime target" to indicate you'd have to hit that creature first, and "opposing spells" to define spells you'd cast against your oppponent but not counting buffs you cast on your own creatures.
And ya, I'm sure this has been done before, but we're all just copying M:tG anyways right? : P