I don’t remember that encounter being too challenging, except the constant danger of being one-shot to death with shove due to nearness of lava and death pits on almost all sides of the arena.
If you are about my problem with duergars, the main problem for me was 9 enemies, that is "a little bit" too much for my party of 4 + Nere who is somewhere there. And yes, flying into lava was a 100% part of the battle
