There are 3 teleporters in this cave, two of which have generators that need to be disabled (though it would be better to clear all 3 before fighting Barnabus). One teleporter is high up, on a ledge with a dragon skeleton, one is on a mid level ledge and the third is at ground level.
It is not necessary to kill Barnabus (aside from a quest to do so, which is part of a larger quest), but like Niner I would recommend giving it a try. There are some moving / sinking platforms in one of the sections, but no disappearing ones, and the required timing and coordination is much more forgiving than the platforms in the depleted ore mine. With the sinking platforms you just have to keep moving, and even if you miss a jump there is time to try again.
There is a dragon skill book by Barnabus, and he drops a pretty nice one handed hammer when defeated, as well as a ring and another potentially good quality random item (and another hammer if you mindread the demon Razakel). I used Barnabus' hammer for quite awhile before being able to upgrade to a better weapon; that and another hammer I found shortly after made a significant difference in the amount of damage I could do duel wielding.
For the platform section, from the walkthrough