Make absolutely sure you go into the battle with saving grace on everyone. This is key. Also make sure to use your boost to break this bastard before his first turn or else he will put a lock on your ability to buff yourself for 5 turns. This should be easy enough by using a single boost physical from one person and a double cast spell from Alfyn as his initial breakpoint is only 4.
You want to use Haanit to keep leghold trap applied. That's her purpose entirely. If you apply it with boost you'll get a bunch of turns on the sorcerer with it, which frees you to stockpile boost and possible get off the warrior divine a time or 2 before needing to reapply leghold. Feel free to also use her to occasionally pop an item if you need to.
Therion is there to assist with breaking via boost and to use SP/BP items on the others as needed. You can also use hired help if you have the spare money and want to.
Alfyn is going to be used for 2 purposes. His first is the most important. You want to get his BP up as soon as possible and use the Apothecary divine on him, then the next turn use concoct to mix a purifying seed and essence of grape. This is going to give everyone 9,999 health and buy you the time you need to set up the rest of the strategy since this guy will get 2 triple cast spells in per turn. After you do this, alfyn is there to break the sorcerer and cast max boost double spells when he is broken. Obviously you can reapply the health buff with him if you get back below max health, but if you play this right, you'll never need to.
Primrose is the most important person here with the dancer/cleric set up. If you get the sorcerer broken before his first turn, you're golden. Otherwise you will have to use her to heal and stall until his buff lock on your characters wears out. What you want to do is feed her a big pomegranate and use the dancer divine on her. Then the next turn you want to boost reflective veil. This is going to give everyone reflect. Keep stacking this up to higher counts (9 is the max). Keep casting it while you rebuild bp for the dancer divine again. When it is down to 2-3 stacks and you have the required bp, recast dancer divine and start restacking veil. As long as you keep doing this cycle you will never get hit by the sorcerer. Each spell it casts on you will reflect onto it 12 times, so when it uses a spell it's weak to at the time, it will also be breaking itself. After a few minutes, the sorcerer will commit glorious suicide.