Basically fight as if it was a real fight left/right hand jab, step back, right/left jab, step back. Don't rush in let him come to you mostly.
i had to uninstall the games update. That big alien just wouldn't ever die and the little aliens were killing all my shields and 1/3 of my health each time. ( usually just getting into the loader, I lost half my health) After removable of the update it became easy to (left or right) jab & step back, ( use the other hand) jab & step back, repeat til he lowers head to rush you may be able to get a jab in but be careful (he is quick) regardless you'll get caught). If you manage to get him cornered on a wall give him quick alternating left/right jabs. He'll grab him with the claw after a few to choke alien ( you'll get hit with acid). Repeat the alternating hand jab & step back technique.
My first play through was on the level below badass so i had to choke him three or four time before he stayed down. But soon as he's beat the level/mission is over and you get the achievement. ( I didn't get the timed achievement cause took me a while to find the technique, I got from a youtube video but don't recall the link)