Aground

So this fight was a bit more of a bear for me....first time I attempted this I got obliterated several times and I opted to let the Alchemist fight him for me (which kills him btw so I'd suggest not doing this or you end any further quests he can provide.) but this unlocked the achievement for me. After playing a little longer I realized what a mistake this choice was, so I restarted (really sucks you can't have multiple saves in a single game.)

So to defeat the "Old One" yourself here is what I did 2nd attempt: I went into the fight with a TON of steak like 40 I think, and I also had some dragon blood and other health items, to the point that I was encumbered. I took the Dragon Cannon from using an Alter Gem on a young dragon. Alternately you could take a gun/shotgun with plenty of ammo. I also used the steel armor. I is also nice to have a fighting familiar to help you. When in the fight, hide behind the Alchemist until all the orbs are absorbed and attack when her armor opens up, which will only be for a short period. It's like a beetle opening it's wings. Once it gets to half health you will be in stage 2. Be ready to get out of the way either run to the entrance to the area or the very bottom and avoid her mega blast as it will chew through your health items fast. Expect to be caught by it a few times thus carrying in a lot of healing items. Once her blast is over you'll have a brief opportunity to attack. I brought a diode wolf to the fight and HE WAS AWESOME! So far he's my favorite for his attacks. I think during stage 2, he did more to defeat her than me as that is where things were tough.

Hope this helps.