Balthazar is easier to fight at the nightsong...

[Fast]

I noticed a fair amount of people struggling with this fight, but erm, if you fight him at the night song, you can just go to another character during the conversation, move all the piles of bones to a ledge, and then when the fight starts use a void bulb to throw them all off, you can also pickpocket his haste pot before the fight, as well as shoving him off the cliff as well.
Altneratively you can trap them all using chests and put barrels on them or whatever your preference is to one shot them. i found the blood of lathanders mace sun beam to be perfect for this as well, whatever you like.

If you dont wanna shove him off you put some barrels on him but make sure they are away from aylin if u wanna save her. ANd use a void bulb to put him near the barrel.