Open up the power house, model industries, and the transverse tunnel. This will allow you all the possible room to run trains without voiding the achievement. The mystery box will spawn either in the powerhouse or model industries so this will give you a chance to grab a weapon you feel comfortable with. There is always a chance if the box moves it can stay on the west side with you. You have the MX-9, Vapor, and the GKS all on the walls available to you. I would suggest grabbing one that way you can always buy ammo. You don’t have to beat round 20 just make it to 20. Save your special weapon for if you get in a pinch. This can be done on casual. If you are having trouble you could always add bots but you shouldnt need them.