This wasn't as hard as I expected it to be - here's how I did it.
Stop reading now if you do not want SPOILERS.
I was a lvl 29 / 30 Lucas, with an AI Reinhardt.
The last boss is a 2 section battle - first part you will need to damage and then remove the armour to his left hand. Nothing too troubling here, but watch out for the 4 or 5 blue fireballs that launch at you - get hit by all 5 and you could get knocked-down, so roll (LT) just before they get close.
After that fairly easy section, the boss loses his arm and dislodges some rocks. My strategy thereafter was to hide behind the bottom-left rock (at, say, 7'o'clock on the screen).
This is beneficial for two reasons; firstly no black goo is generated here and acts as a natural 'choke point' as the enemies circumnavigate the rock to get you - so get the sword ready! Secondly, it stops the blue laser that he fires from tearing you up - whilst it's not deadly as a one-off hit, it's cumilative effect is deadly and this Cheev needs you to avoid damage where possible.
So, I leapt out from the rock and went to my idiotic AI's aid, as he decided to stand in the middle of the battle-field and try and Rambo it. I'd thin out the middle with blade-dash attacks, try and hit a few green orbs for health top-ups and then retreat back to the rock. You need to keep your partner alive as it takes too long to revive and you'd get knocked-out in the process.
Once the boss allows you to assault him , open up on him and then return to the safe-house and rinse and repeat.
Remember to self-heal! This allows you to help the AI whilst taking damage, as your being healed at the same time.
After 5 or so waves, the boss was defeated and the Cheev pinged! Good times.
Good luck!