Need help defeating Rexoskell

I have been trying to fight this damn octopus several times, and I can't defeat it no matter what I try. What sadistic madman put a skell boss in an area that you can't get to in your skell, not to mention it is both topple and stagger resistant. And because it is a normal mission boss rather than a character mission boss, there doesn't seem to be any easy mode available (I have died to it 5 times with no easy mode pop-up). Some help on how to defeat this thing would be appreciated