Very straight forward, no hidden steps or anything. You can do this on Casual if you want as well, just make sure there are no modifiers active.
The Galvaknuckles are located in the truck on Cul-de-sac, behind a 1750 point doorbuy. The Galvaknuckles themselves are 5000 points. Get to them as fast as possible, without elixirs you should be able to get them by round 4 or 5. The rest is pretty easy, just only melee for the next 115 kills or so. Make sure to not be too greedy, don't put yourself in any tricky situations, and pop your specialist weapon if you need to. Galvaknuckles are a one hit kill well into the early 20s, so you have a good amount of room for error as well.