wow, i think the shotgun is laying under the radar for most people. I went from my pistol to the DB shotgun right off the bat, i also used the perks that help out the shot guns reload speed while i maxed out the DBs damage, clips, and fire rate. I then got the heli and the turret and some builders. After a while i bought combat shotgun, maxed that out, and then maxed the heli and the engineers and it became really easy for the rest of the game