For me, the easiest weapon and the fastest to kill with is the Conqueror Type-E pistol under Firearms. Very powerful weapon for short-midrange, and a very fast reload. Seriously, this gun gives the Halo:CE Pistol a run for its money with how strong it is!
Game mode doesn't really matter, as they both last around 5-7 minutes each. Thankfully it's cumulative between both.