Be sure that explosives detonate under it's shell thing right next to or under it's nard.. Also, to save a bunch of running around defending yourself while Gonarch seems to refuse damage, approach the end of the slippery part very slowly so as to not end up in the pit below. You'll be dodging a spitball now and then but your character won't die if you remain there, long as you've decent health points to start. If it still seems as though things are going nowhere restart from a previous save, preferably one prior to Gonarch's Lair. Also, if you restart before the Gonarch chapter you may want to conserve your explosives inventory more than you did the first time through. Good luck!

Hmm, I'm standing at the lip of the pit at the end of the tunnel. Should I go further? There's a shelf of some kind with some shining crystals on the other side of the pit and a bit down. Should I jump there? I haven't noticed any "slippery part"...

Hop down there, yeah.

You're not where you thought you were, which is where I thought you were cause you said you were there already, which you weren't.

Ok, that was crystal clear, or should I say diamond clear ;-)... I'll try moving a little further. Thx!