But in D&D Next, putting all of the target creatures attacks under disadvantage for the next round may be enough (or even ideal) as it shakes off the magical effect.
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I actually like this solution. Very simple to implement, and rarely is someone going to choose a spell just to cause disadvantage, but it does provide a useful effect. You'll always go in with the hope that a spell succeeds, but without primarily banking on 1/2 damage or something.
It doesn't break immersion for me if a guy spends a round struggling to resist a polymorph spell. It actually makes more sense then a guy getting hit with a powerful spell and shrugging it off in less than a second. Reserve "Your pitiful spell is useless against me!" for situations where immunity comes into play.