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. 

One other thing, she has a remove debuffs skill so you want to watch the cooldown on that because if she uses that right after rocktooth's skill procs then you have to start all over. You can get her to use it by hitting her with a "decent" debuff (i.e. decrease def, atk, or speed). Right after she uses it hit her with heal reduction. That boss doesn't have a ton of hp so dropping her in 3 rounds is definitely doable with some decent damage dealers. Best of luck.

or if you have the champs (High Kathun, Alure, Nethril, Psylar...) just slow her down and use raw damage.. she can't heal if she never gets a turn ;)