I also would like to know, though I'm playing as Zero. A quick Google search revealed that all Threshers are weak to fire, but I don't have any fire weaponry.
You can try and buy some fire weapons...
I was level 16. Sniper rifle with slag followed up with fire smg, he was down in a minute. Ignore the robots until they get close. The Thresher was between me and the robots, so I didn't have to worry about them.
i play as support siren so it was even harder for me, what i did was go behind the building use that for cover, pop out empty your clip let your shiield regenerate then repeate its a slow process but it worked and when i needed health i just phaselocked the robots and killed them for health orbs
I had to lure it to a spot where it couldn't get within melee range. I used cover to let my shields regenerate but it still used damn near all my pistol, rocket and sniper rifle ammo to kill the damn thing. I died at least 3 times, it has been the single hardest fight I have encountered thus far and I strongly advise that you do not try to go toe to toe with it.
Threshers in general are a major pain in the ass, wait until you have the delight of running into wormhole threshsers...
\\ .// //\\ || //\\ ||\\ || |[ You cannot understand the magnitude .\\//. \\// ||\\ \\// || \\|| |[ of how cool of a story this is - axerer6
ok, i had a similar problem with this thing (problem being ~12 deaths), but i found a way to easily kill it. first off, i was Siren and didnt need to use phaselock for the damage. ok, 1. aggro thresher 2. kite the thresher to the leftmost part of the arena according to the map 3. if you run around the corner of the large hanger-type building, the path is too narrow for the thresher to follow. 4. the thresher will continually pop up and down to try to maneuver through the gap, but he cannot. 5. shoot the ever-loving crap out of it with fire weapons (i used a fire sniper), and use a tesla weapon when it regens shield (again, i used a tesla sniper) 6. back up when it attacks and move around the corner when it fires the spikes 7. repeat steps 5-6 8. win.