There is an island named Snidhall north of Ard skellig which inhabits an archgriffin with unknown strength (right now I'm on level 33). It protects a hidden treasure.

basically I do not have something against strong enemies but to have a treasure guarded like this is a bit pointless for me. Does anyone know the purpose of such a strong guardian?

Thanks

The treasure is absolutely worthless too. It's just there for the challenge of fighting an overpowered enemy. At level 33, beating the archgriffin is doable. I did it at level 27, and at that level, any attack the archgriffin did to Geralt was a one-hit-kill, so it was a long drawn-out battle of hitting once and rolling away (and using that one decoction that uses all your stamina for strong attacks, but does guaranteed 10% damage to the enemy).

So if you don't want to fight the monster for the sake of fighting the monster, it is entirely skippable.