i just go through story, but i focus on scouts/shocktroopers and i leave snipers and lancers 1 lvl lower, then comes engineer. Works great for me, because i just love to run to the enemy with my scouts and shoot them in the face from close range, snipers were great distraction for the enemy on the recapturing the bridge mission - jut get one of them onto the tower and shoot at the Ragnite "barrels", after that tanks will focus on sniper(10% chance to hit him or lower), lancers are pretty obvious to keep them high, and i use engineer only for repairing tank and restocking ammo(i just keep her behind tank and repair when i need).