Rifles are your friend. Especially if they have the Demon's special and hit% or if its name is 'Frozen Shooter'.
The FO doesn't really need to do much more than S/D/J/Z really.
A ranger of that level should kill everything fairly quickly with a decent rifle and Shifta. The Deldoggies shouldn't be much of a threat if this ranger happens to be a droid with freeze traps, or if he has a Frozen Shooter. Bring mechs to finish these off a bit faster.
But most important is that you learn the layout of the tower, where and when the monsters spawn. Most of them can be shot from behind the safety of a wall or some other object (this includes most of the Delbiters
), and at maximum rifle range the plant things do not attack you yet even if you're hitting them.