Got to final boss which is proving to be a bit of a pita. Multi-phase with initial easy stage followed by dual phase bigger boss.
It involves just avoiding attacks until you get transported to a different realm where you get swarmed by smaller mobs. Each kill gives you a buff to damage for when you choose to go back through the portal to the big boss. However, you also take constant dot in the other realm - so bloodwort and hearts are needed and you can die very quickly. Seems like quite a clever mechanic, but it just makes it a fairly dull, consumable sapping annoy-fest. Will give it a few more goes before binning it.