@Motorex: I played quite a bit with friends, and once I started focusing on grinding seriously, I only needed about 2.5 million. Which took about 2 and half days. Obviously I was getting about a million per day, and was very adamant about the timing of it.
As for the Trooper, try using the Rockets, several of them are tracking rockets, though it wont say it in the stats, it will say it in the description. Several of them (I think Nungus?) will track enemies. The missile launchers werent that good for him, so I used rockets, or his Grenade Launcher, which was very potent at all levels.