Make your way up Mt Gelmir until you get to Volcano Manor.
You can then join the gang there and hunt for a few bad Tarnished before you can get to meet Rykard (you can refuse this and fight through the Manor grounds if you don't want to hunt anyone, but that's not what I did).
Once the hunts are done, talk to the lady and she'll transport you to the boss room. Make sure you pick up the serpent spear near the entrance and use that against both stages of the boss.
Pro-tip: To make this way easier, die quickly first time so you can pop out and upgrade the spear (I did it with a +3), and also equip it and spawn the mimic tear summon and it'll use the spear as well.
Enjoy!