Same here. Maya finished Fink’s slaughterdome, got her Hails, and finished the Torgue DLC. BA Rex took three tries and it was very close on the third one. Then Piston himself took four attempts. After the initial try Piston camped the gate for a while. Finally he left to wander the Arena, so on the final attempt I could dodge his goliath-leap while Ruin applied slag to him, then spin around and Beehawk the crap out of him. Ugh. If only this was an optional end-of-DLC boss fight like Jackenstein, but sadly it’s the gateway to Pete the Starter Raid Boss instead so every character will have to do it… 
Re: fighting BA with Krieg, I found that to be much more enjoyable than with Maya. Granted it was TVHM not UVHM, so I’m not sure the same tactic works. He shot his Kerblaster whenever he could, ignored the nukes and used RtB to heal right back when health-gated, and wore it down. The nukes never fired with a cadence faster than about 20s, so there was no real danger of dying. It did take a while though.
Maya found that the nukes were often destroyed by an active Storm Front, which I thought was interesting.
Her key to healing in that fight was to have a SF active as much of the time as possible so a Grog swap would fill her health back up quickly (the Hail’s 2% isn’t fast enough; it’s useful for sustained healing but not burst heals).
Anyway, back on topic. Maya finished by taking out her frustrations on Mr. Snowflake, Esq. Still no useful loot, not even enough eridium to finish another ammo category. Her bank account has mostly recovered from all the New-U fees though.
Earlier in the day Krieg did a bunch of Zaford-Hodunk quests, good for about half a level (now 41.6).