I would see it as a 2v1 or 5v1event. The Boss Mech would have enough health to combat the collective fire power. It could be loaded with multiple independent weapons that all fire on they own in different directions to void blind spots and flanking. It could be a server wide event in which there are an "army of rogues" where there is a single massive HP bar, and each Boss Battle depletes the Army's health. Bonuses awarded for beating a boss, but also globally for defeating the army. New areas would need to be made. Team synergy will need to be considered by players too. There is a mountain of opportunities here. A new type of entry ticket. Or even better, the monthly subscription to a 'pilot licence' giving access to this kind of event and more.
I've got pages of ideas and notes that I could submit for some new design ideas, but the forums are broken on my mobile device (I have to write a reply in notes, then copy and paste it to the forum. If I try posting directly in the forum's, I get double inputs and jumbled words... But the PMs are fine... ) and I have no interest in using Discord so my ideas are kind of sitting in limbo...
This is very much along the lines of what I was thinking - a large, multi weaponed very tanky boss that would take a bit of coordination, and would definitely make the MD bot very useful in the rotations. Use that big open arena map, and it'd be a fun dodge and weave and return fire fest.
Or, like ToonNinja suggested, a server, or clan-wide total hitpoint race to take down a truly massive boss together. Something like the clan boss in Raid shadowlegends.
Another thought I had would be a good old fashioned capture the flag mode, or some sort of fortress assault mode. So many possibilities.