To beat Rahu III, it is best to use either the Dragon gun or the Gatling gun. Stay away and let Marcia and Harry take most of the damage.
To beat Ray Legend also known as Ray Mk II, he has a very low defense but can deal alot of damage fast. Use a speedy type of leg mod and use tons of pods and bombs. Bomb types you should use and best against him are, Tomahawk-Which ever one you want, Freeze, and Wave. The best pods are Speed-Which ever, Cockroach-Which ever, Wave, Freeze, and Double Wave. The guns I'd recommend are Gatling, Dragon, Wyrm, and Shotgun-only if you use a fast and small Robo.
Swift Strategie, the Swift is very fast, go figure, and with a high damage gun and a speed leg upgrade, she could be very powerful. I recommend hit and run tatics with the Swift using the Shotgun and a fast leg upgrade of your choice.