The Monitor is perhaps the most annoying, backstabbing, relentless piece of crap in world of gaming.  However you can't kill him, so don't waste sniper, rocket launcher or whatever your weapon of preference may be, trying to kill him.  What is fun though, is ruthlessly massacring his bodyguards, The Sentinals.  The best way to do that is with plasma grenades, a shotgun, or with a needler.