The easiest way to do this is with someone who can Hack. Have them hack a Geth... then when its on your side punch it and kill it!
Specifcially, do the side mission
N7: Wrecked Merchant Freighter which is located on Neith (4th planet/Amun system/Eagle Nebula cluster). Apparently there are 180 Mechs... plenty hack and melee.
More info:
http://masseffect.wikia.com/wiki/N7:_Wrecked_Merchant_Freigh...