In the arena, just toss the Megaturret Glove(v5) while his 3D is attacking you. Then use the Plasma coil(v3 at least) while he uses the lava laser, you can avoid it easily by moving far back. Just use these two and he'll die easily.

When he leaves use the Plasma coil to kill the big machine robots then accompanied by Agents of Dread(v5) to kill the soldiers. (I only used these 2 weapons til I reached Nefarious again). But you can use the Heavy Bouncer(v5), if you want to kill them faster.

Once you reach Nefarious, use Megaturret Glove(v5) again, and Splitter Rifle if you ran out of Plasma coil ammo but it will take longer to aim. Use the Agents of Dread(v5) to accompany you and the Heavy Bouncer(v5).