The lamb custodian has earned itself quite the infamous reputation in the past. With him getting some hardcore stuns in and a silly spell that is the equivalent of your older brother telling to stop hitting yourself while hitting you with your own fist!
So level 6-10, let's talk tactic!
First of all, just don't block, because if it launches its spell on you, you're stuck damaing yourself quite hard for 12 turns, even if you stop blocking.
Second of all, the guy stuns and can deal a good amount of damage with his stuns. My best advice would be....just get a full purple tank gear and you should be fine hp wise. Another option is just use scrolls instead of potions. This way you can maintain yourself at almost max hp all the time without wasting valuabale hundreds of hp from potions just to stay above the stun safe zone. A slot or two of vampires won't hurt either.
Third, bear him (level 9+), or great thirst for blood. Give him a taste of his own medicine and stun the hell out of him!
Finally, do not use mounts, or wraiths. Thats a no-no for every custodian, as they'll recover 35% from every ennemy kills. The only acceptable mount would be the Arhas, as you'll fully recover your hp in return of him regaining 35%, decent trade off. If you realise you won't make it by a very slight margin, use as a last ressort a strong summon of your level or below, as it'll grow to the wraith's level if it is higher. If your Roc summon is still low level, it is a pretty good option. Just make sure you don't use it too early, if it dies, you're screwed.
If you still can't do it, buff up a little to make quicker work of it, a rage, a set buff etc...
If you're still missing something to kill it, contact me in game, we'll get there!