i had that problem with Piston. luckily you can shoot through the barrier there. which, IMO, was terrible gameplay since it was pretty much instant death if you went into the ring. one nuke, insta-dead. garbage. made the whole Torgue DLC just feel plain awful.
i havent played this DLC as yet. sounds like more of the same. hopefully there will be a glitch to exploit like the others.
Nakayama was pretty easy for me and I had it on TVHM, the boss you had to fight before you fought Nakayama himself was hard, all I know is that you're supposed to shoot the two things on his back that was electrified, then shoot the thing he exposes when he throws his arms to the side and straightens up to do some sort of attack. Now Nakayama all I did was shoot the hell out of him as he was falling down the ramp he creates