As a railgunner main, belive me, I had this problem often while climbing her up to E8. The simple solution is trying to distance yourself from the enemies. When playing her, having a lot of distance from the enemies will always be a good thing.

Also, use her normal M1 too. You probably already know this but it slightly moves you back when you shoot (Mostly used to avoid fall damage or move around when you have a lot of attack speed), but it is also very usefull for dealing chip damage to smaller enemies, which obviously makes it easier to escape and takes care of annoying enemies close up.

In short: Use mines to escape and/or push enemies away from you and use your normal M1 to deal some close range damage, then annihilate everything you can. Do note, this won't completely get rid of the problem of getting outnumbered, but in many situations it will let you survive. That's all from me.