my strategy is to focus exclusivly on one crystal at a time.
each crystal does something different. give health, mana, status effects, reflect, etc, so ignore the middle one and focus on the cristals recover that heal and the one that inflicts status effects. also, have one of your party members for that quest learn silence
after that, ist mainly up to your playstyle. if you use wizards, take the protags sister, she has an ability that does more elemental damage
if its phisical, take gargan, he/she has a stun lock
hope this helps