I’d just like to know once and for all what this means, since it’s been hard to find clear answer anywhere. Exactly how do the enemy masses become tougher to beat? Do their health and attacks rise or does the game just spawn more badasses and other tougher variants? Or perhaps both? Does this change between any of the games?
Thanks in advance! 
Health and damage of enemies rise.
And they gain health regeneration too.
It does also spawn tougher variants in place of weaker ones as you rise in level & gameplay mode.
Okay thanks. Btw I just thought of asking in addition if any of you know the algorithms for these changes? How much the x amount of players increases the health, damage, health regeneration and propotion of tougher variants?
Are you talking about when other players join your game (or you step up to a raid boss) or when you make the jump to UVHM?
Of course I’m talking about players joining in my game! That’s when this notion pops up.
I’d also like to know what exactly happens to the Creatures of Pandora when someone joins my game? How do they Grow Stronger?
More players = more enemies.
More enemies = more chances for spawning badasses etc.
More enemies = more XP and loot
More loot = more chances for getting legendaries etc.
No changes to enemy health just more of them?
Not sure about the health. I think the raid bosses get more health the more players you have, but I hadn’t noticed regular enemies being harder to kill (apart from their level and the different playthroughs.) If you’re on 360 we could check it fairly easily!
Won’t really be on until Monday
Snarky much?
Enemies gain +100% base health per player. So when you have a full team, enemies have 4x the amount of health they would in single player. I do not know if they gain increased damage, but I suspect they might. They do not gain increased health regen or anything of that nature, although in UVHM, since their regen is based on max health, in a sense they do. In addition, in UVHM all enemies will scale to the highest level player, making them MUCH more difficult if the team isn’t equally levelled. Not sure if this health formula applies equally to the Raid Bosses, but it is the way it works for standard mobs. In addition, in certain places during the story (rescuing Lilith, after destroying the beacons with Brick, etc) there are guaranteed baddass spawns which increase in number depending on the number of players. And of course Varkids have a higher evolve chance as well, and Tubbies spawn more frequently. With all of these changes, however, the game is still easier in co-op because of the division of enemy aggression. I hope this answers your question.
All enemies, or just raid bosses? Source?
I’ve heard that widely referred to as the formula specifically for common mobs. The source I would cite specifically would be Broman and Teawrex talking about how speedrunning with two people is faster than solo even though enemies have twice as much health. He mentioned the same for a full team and 4x as much health during his team’s AGDQ BL2 speedrun a few years ago. And [quote=“stuarthome3, post:14, topic:1090663”]
Not sure if this health formula applies equally to the Raid Bosses, but it is the way it works for standard mobs
[/quote]
To quote myself. A friend of mine who was fiddling with some health formulas a few years ago referred to Terra’s health as 4/3 its normal amount with two players in the game, but I do not know how accurate this is.