It seems to me that 1 way to beat it is to constantly applies states reducing its hp regen, while keeping decent damage output. When its reinforcements come, just destroy them as soon as possible.
As for preparations, apart from stockpiling top level gears, acid/fire shields and hp/hp regen addons, I've to sell all the head addons(stat masters) to replenish the ammo of all party members(as I failed to find any ammo station) and acquire enough special ammos, as level up bonuses don't seem to help me when I'm about to fight the final boss.
I don't know if level 16-17 is way too high, but I think that's all I can do for now
