I was able to kill him at level 7 using a rogue and a summoner. Dropped an uber sword, a dagger called Toothpick, and some other unique items.

whats better kill or save him ?

how i can save him?

considered saving the dragon but his name is slane so I slained him

Is this dude like an OC insert from crowdfunding or something? I find it a bit strange how the MC kind of gushes over him.

Rupture Tendons (Scoundrel skill) absolutely wrecks Slane. I applied it with my rogue, then Slane used Dragonfall to attack my max-range archer before using Fly to get back into melee combat with my rogue. The rupture knocked off around 2/3 of his health when he used Fly.

i did it on Tactician thanks to the Dome Of Protection.6000 xp and good loot..Then again tactician isnt meant for an Origin Character other than custom.Their source powers are awful other than Fane perhaps.
Magic doesnt work well on Slane because he has permanent Soothing Cold..Physical damage is required.

If you save him and give him his source back he'll kill all the shrierkers at Abandoned Camp area.

Killing him gave:

6000 xp

Unidentified Epic Dagger (with various names, one I got was Longshank).

Unique Sword - Fang of the Winter Dragon

2x Unidentified Rare Rings

Skillbook Dazing Bolt

Medium Flame Rune

This is what I found. Not killing; - He kills two Shreiker's for you. - He gives you not much more than 2k exp and some loot, one of which you can pick. - You generally just feel like a good person. Killing; - You save a purging wand. - Upon death you gain 6k exp and much better loot then what he offers as thanks. - Unsure if it's a bug, just a thing in the definitive edition, or normal, but I gained +5 attitude for all party members including The Red Prince and Sir Lora. (Looked by right clicking and clicking 'examine') I checked if perhaps I gained more attitude if I went with freeing him but I actually got nothing for anyone. The fact you also never see him again in the game nor does he actually do anything for you or the story other than kill things that aren't that big of a problem, assuming you're not dumb with the wands, the devs really made it not feel worth just letting him go. Plus there's even a steam achievement for killing him. Even if he was just an NPC you could chat with again, romance, or simply have a shop would be nice but no. You could argue that the loot becomes useless later down the line so why bother killing but then you could just sell it, plus he gives you more than double the exp than saving him which is tasty.