The game is alot easier if you set your tactics to where all your party members focus fire the same target...of course in some situations you will manually have to cast crowd control spells..or you can even set tactics for that..but I prefer to do CC manually.
On your characters tactics page go to one of their offensive tactics...which is on the right and then on the left set it to..Enemy->Target of-> Controlled Party member...do that on every party members offensive abilities..and you will burn down enemies in no time.
Also, if you are using Wynn..you probally want to change her tactics..because I found on the default settings she was burning mana way to fast..so I lowered the health threshold of when she would heal party members and her self to >20% for heal and group heal. Of course..you might want to have a tactic slot devoted to your tank and maybe having a heal tossed his way at like >= 50% health...since he should be holding all the aggro.