Either this spell is broken or it requires so many pips to do something useful that it's a waste of deck space.
Using one or two pips, I have yet to see a single instance where I actually lost aggro.
It was particularly annoying when I opened a battle in the Master's Tower where my "teammate" was pretty much insisting that I go first and grab all the aggro every time. I got a power pip, threw Subdue to try and shift one of the Bone Digger's over to my "teammate", and ended up with all of the aggro from both enemies anyway.
By the time you get this spell, you're fighting Rank 3 enemies. If it takes three or four pips or more to get a successful Subdual, the card is a waste of space in my opinion.
By the time I've waited three or four turns, I've been laying traps and setting up blades and possibly buffing my minion. Odds are good that I've had to throw a heal in there someplace to keep up with the incoming damage during those turns.
At that point, I'm not going to want to throw away all of those pips just to attempt to POSSIBLY get one enemy to shift over to my minion or teammate. I'm going to want to lay down a big single-target hit or a multi-target hit and try to eliminate the enemy entirely.
The best "healing" is preventing the damage in the first place. The best way to do that is to end the battle, not prolong it; especially when prolonging it uses up all of your built-up resources.
If Subdue is working as designed, then I'd say the design needs to be revisited.