My facourite gun is the sickle, but you won't be getting that any time soon.
The AR-23 Liberator isn't a bad gun to start with, with bots, it's all about aiming for the weak spots.
I've not used it in awhile, but the SMG-37 Defender was a nice upgrade from the liberator, despite it being a SMG.
Shotgun wise, the SG-8 Punisher and the SG-225 Breaker are solid choices, despite the recent nerfs. But my favourite
But disregarding all that, I would say the best thing to do is invest in the AC-8 Autocannon strategem. That thing will kill 90% of the enemies in game.