An option to having bullet sponges
Hi so here's a thought instead of making beefy npc's why not take away player advantage?
Example if you play a story mission on normal it plays pretty much as It would now just adding a few more npc's per player.
Then when you go to the hard setting take away the bonuses on the gear so no more brand set perks and angina just add more npc's based on number of players.
Then on challenging no more gear perks and lose the weapons perks as well( could even be harsh and only take the positive perks of the weapon mods and keep the negative) and once again npc's by how many players.
Lastly legendary would have all of challenging but have skill cooldown increased slightly and if that wasn't enough you then could up there armor/health by 10 to 15% and there damage 5 to 8%
Just a thought on an alternative to npc's that need 1000 rounds per player to kill.