I did this on my own on:
level 7 battle of the bulge - grain processing (the first level on hard you can do).
I am level 40.
It took my 20 or so tries going for this in total, it was just a matter of learning the spawns and taking it slow and making sure to take the right weapons of course.
I went for:
vicious 110% American (26)
ultra light ripsaw (28)
beer hat (8) obviously
reduced emissions jet-pack (40)
rodeo clown belt (39)
In about the middle of the level you will be on ground level and flying robots will attack as well as ground level robots and then two large robots will appear either side of you.
This is the hardest part of the level in my opinion.
Firstly don't stay on the roof on the left for this bit, you need to go to the right and wait for the big grenade launching robot to appear on the left on the roof, use you secondary weapon to kill this robot and the air robot's quickly after that you can settle on the roof on the left and the other big robot on the right should then appear.
In other spots on the upper levels when the air robots appear try to stand still and shoot to your upper left and you should find a safe zone where they cant hit you with their red bullets.
The boss is easy just stay in the middle and look for the gap in the oil each time it tries to flame you. Only one robot will come on the ground for each flame the boss does.
Be aware that right at the end after you kill the boss another ground robot will then spawn so watch out for that.
I wanted to add my own guide as nobody was really talking about what weapons to take with you, what bits are hardest and what to do if you were not rank 50 etc. as obviously being rank 50 will be the easiest time to go for this but I wanted it out the way before rank 50.
I hope this was of some help to somebody.
Glad to get this done, now I just need level 50 rank and 10,000 robot kills.