First time you meet Seath the Scaleless he`s immortal, so you had to die to beat him later. But i knew there are some method to skip this. So any tips with it?
There is a (probably unintended) shortcut by jumping down from the ledge of the second elevator in the Duke's Archives onto another floor. You need an equipment load of less than 25% (fast roll) to do this.
I have found a good video explaining how to do it:
Take the elevator up. Once on the top floor, activate the lever to start the elevator down again. Quickly run onto the floor and from there roll down onto the ledge on the right (facing the floor) of the elevator that is descending. Once you see the first railing appear, jump down toward it to attempt to land on the floor. You have then skipped a whole lot of the Archives.
There is no "official" way to skip the first encounter with Seath, you're intended to die there.
As mentioned by Ben and Jubatus in the comments, you could also wear a Ring of Sacrifice during the first encounter with Seath to nullify the death penalty and keep your humanity and souls. That way you will still be treated as if you had died and can continue the level in the intended way.