For EM Refire Booster best choice is 22222. Very good AoE bc of T5B. And one of the best crowd control for engi. As for Breach Cutter 22 in last two tiers is the best choice for crowd control and annihilation. Also good against swarmers and naedocytes. Breach Cutter for electrocuting praetorians and for crowd clear (if need). Stubby for killing praetorians. Weakspot of this build is oppressors. Only way to kill them is S.S.G. and pickaxe (also stubby, but you'll spend more ammo).