This achievement was really fun, for me! I would recommend using a Glock, and skip on the shield, because it's just going to slow you down. If you're good with burst fire, the Glock will get you maybe two or three kills per clip, aaaaaand *drumroll* YOU NEVER RUN OUT OF AMMO! Hells yeah. You've practically got an extra submachine gun at your hip.
The Desert Eagle is also a decent, and a lot of fun, but if you're going to use it at distance, be sure to add the extended cartridge. Bigger is always better.
Get it in campaign or get it in T-Hunt, but the campaign is way more engaging.