This is my fist solution, so go easy on me!
I found this achievement really easy to get on the mission where you have to go to the dog park (once you meet Sam's friends). A spawner keeps spewing out OD, and there is a round grind rail, in the centre of the park, close to the ground.
Start grinding in a circle, and keep meleeing until the chievo pops.
If you have a melee amp, it makes it a lot easier too.