This is a result of poor game design.
I think the bosses are supposed to have actual mechanics, but they had to disable them because the engine optimization is terrible and lower-level PCs can't handle the amount of players who participate in them. Well and that the scaling just doesn't scale up high enough, a problem that all the events have.
Let's take Tequatl as an example, since there was some coverage about that fight before the game came out.
You have Tequatl, who's dangerous to fight personally because he's a frickin dragon and throws AoE's around.
There are cannons around him that can damage him from a save distance.
He summons Walls to block the shots, Bone Tentacle things to knock people off the cannons and other minions to kill the players who are further away. Including big suicide bombers.
There's a megacannon that charges and knocks him on his ass for free damage.
With this, there are multiple things players can do to contribute: Attack Tequatl directly if you can dodge his AoE's, kill the walls and tentacles, go on the cannons to deal damage to Tequatl or protect the other players from the suicide bombers or normal mobs or protect the megacannon crew from mobs.
Wouldn't win any game design medals, but it is pretty reasonable and requires players to coordinate. in theory.
In practice players are going on the cannons and the others just murder Tequatl with there weapons. I don't even know if the megacannon even works, didn't figure out how to fire it once it was charged. (If it even gets that far)
Mobs just get obliterated in the Zerg and if there are enough players the lag is bad enough that you can't really do much anyway.
It's not that the game design is completely terrible (although it has some issues), it's just that the implementation doesn't keep up with it.
As far as the Fire Elemental goes: I think the main problem was that many people ignored the spawned fire elementals, they tend to create multiple lava fonts and if there are enough of them, you don't have enough space to dodge.