He's tough, but the clue is: Let that mercenary tank merry first. Then the monkey will run right for you. Keep moving away from Merry and kill his monkey. Now merry's shield will dissapear, you can hit him now, or just leave ur merc fighting him alone. (Best option is to stay away from him becuase he can AoE). Then when he summons that damned monkey again, lure that monkey asap away from Merry and kill it again. Its shield will dissapear again and you can choose to hit or run again. My merc died when Merry had only 30% hp left. That isn't a lot and i finished him off with like 3-4 hits more. So he hits hard, but has low def/hp.
That's about it

(Oh yes, im an alchemist, and my potions helped me ALOT! Specially the healling potions, they healed me for 25% hp, and thats alot when you have 250k+hp while fighting a solo boss

)