Merc with all points in assault. No mob in the fight with the "boss"
Ok, I assume you mean Arsenal tree. So you should be using Mako, and be using high velocity gas cylinder?
Not sure if you have any interrupts at your level, but if you do, make sure you use them on heavy hit specials.
If you are doing all this, and you are using the correct skills (tracer missile A LOT hehe) Mako should keep you alive. You should be getting your Aim as high as poss btw (which I'm sure you knew, but check anyway),
and if you have your shields, use them too.
Without seeing your skill tree selection, and what stats you have on your armour/weaps, I don't think I can suggest any more.