im assuming your thinking about taking on the harvester, if this is the case then you either need a insanely strong warrior with high damage weapons or a mage class with powerful spells,there is a happy medium with a warrior/mage class but i can tell you from 30+ try"s it is frustrating man! the harvester switches from magic resistant to physical resistant when you do enough damage so your always running and healing with little attack time,the party is for the most part useless unless you pre set their actions in the battle menu before the fight.
i used the warrior i had saved that went through the whole first game and most of the dlc and i still got slaughtered over 30 times,finally i lucked out and managed to hack him down to the first cut scene with a bunch of 2 handed moves (the one shot massive damage move and a bunch of others) just keep at it and don't give up it can be beat.