No trick dude, you just need a champ with heal reduction debuff. Go sit in stage 5 of campaign for a bit until you find a rocktooth (might take awhile depending on what difficulty you are farming) and give him a bunch of accuracy gear (gonna want to give him boots with speed primary stat to speed up his turn meter too). Make sure you have someone who can rez or heal really well in your squad because you will have to tough it out a bit until his skill procs. Just tank it as best you can and once the skill procs go all out. Rocktooth's debuff lasts two rounds but because of the way debuffs work you actually have 3 rounds to drop the boss.
Or you can just kill her with raw damage. Since her attack isn't great, and her minions are squishy too, have no or only 1 defence/hp champion. Get as many magic attack champs as you can, (you starter will work great!), then fill in with your void/spirit champions. Then just quickly kill her. Dead bosses can't heal. :)