How disapointing is it to read ppl complaining about having to kill him or blaming those who don't have problem with killing him...
Just kill everything that deserve to be killed, like all the magisters, and this weak lizard who is an insult to the powerfull dragon's race !
You're not here to grow as a merciful hero, you're here to rise as a GOD ! Good and Evil are meaningless to you, so don't think that way !

A consistent way i found to kill him in lizard knight form is to give him the wand, and while the menu option of "end" is available to the talking character, attack him with a second one. That way you will get the 2100 xp for helping, he will then transform into his lizard form and immediately be aggressive towards you.

Update:
They patched it. You cannot agro him when in conversation so you can't get both exp. I past on tactician and I had to agro him before giving the wand, otherwise he would just fly off. He kiled the first character in his first turn and 2 in his second.
So I restarted and dropped a spare death barrel as close as possible on his head. Didn't kill him right away but when flying up to me he touched the deathfog and I got 6000 exp plus some very nice loot (including the one handed Fang of the Winter Dragon sword :D)

Everybody is in the comments complaining about having to kill him. How about you murderhobos chill the **** out and just talk to the poor thing instead of being hell bent on grabbing every last point of experience.

You can get both XPs if you give him the wand and then attack him before you hit end. He will stand up to attack, and will wait until you're done talking. Kinda cheese, and he obv won't help with the final fight or anything, but you can have your Slane and eat him too.