An unfortunate part of this is that the Healer's enmity is controlled in part by the rest of the party. If they get themselves hurt more than they should, you have to soak up more hate. If the DDs get close to taking hate they can just stop attacking. A great tank should be able to out stripe any other individual (though disorganized attacks on groups can screw them over). But Healers... all you can do to mitigate is procrastinate a little.