Veni, vidi, vici.
The final battle against the twins was a cakewalk, and was actually easier than the penultimate fight with the half-dragon delegation. I cleaved through the twins bodyguards like butter, and then targeted Isair, who took bonus damage from the lightning of my 2 sparking throwing axes. Funnily enough you only need to get one of the twins to 'Near Death' to win.
I used an all human party and didn't level squat and/or use mule characters to see what level you'd end up at if you squeezed every last bit of XP from quests/optional fights. In the end everyone managed to reach level 16 just before the final fight, which sucks for casters, particularly my multiclassed druid who didn't even manage to get level 8 spells. Not being able to enjoy level 9 spells unless you manipulate the lousy 3E XP system is kind of a bummer, I always feel like a bit of a cheater when I level squat.
You can't really level squat anyway. After a while you stop gaining XP due to being overleveled for the mobs you were fighting. That is why you play with +LA races because the devs didn't take the LA into account when assigning XP. Even with, say a party of Drow, you will end up around level 16 with them.
The half-dragon fight is bad simply because you start off in a bad position. It is one of the problems with IWD2. It likes to put your characters in bad positions for a fight, like the initial one with the priest of Bane when you start Chapter 5 (i.e., get to Kuldahar). My usual 4th and 5th slot wizard and sorcerer ended up on the front lines instead of being in their usual positions in the middle of the party when stepping off the airship.