Two bosses I feel are just overtuned...

A few things to note about the fights is even if you ask for a carry, you as host need to turn off level downscaling otherwise the fight is as hard for the overleveled person (assuming they don't have very good gear). Also additional people makes the game vastly harder for certain bosses like the molten vault boss because all monsters get a huge HP buff. Each mainline campaign boss has a kind of theme too that you should spec towards, e.g. for Geonor get cold resistance (just like Merveil from PoE1), for Jamanra he's mostly lightning with fire, for Viper, get any poison and chaos rings and charms you can. Very generally, movement speed helps a lot - 20% on your boots is enough to make them all much more manageable.

Jamanra I found to be the slowest and second easiest "main" boss (the easiest being act 3's final boss who is the size of a barn). The main things to note about his second phase is there are no adds so you need to be careful with your remaining flask charges (or roll a flask that gets charges per second so you eventually refill) and that there's a lot of stuff on screen in both phases but they're mostly "static" (don't stand in the tornadoes).

Viper is definitely the hardest of the campaign. She's fast, has a lot of HP, deals chaos damage that most players won't initially have defenses against, the arena gets smaller, but the tracking on her attacks is bad (as in the chaos projectile barrage she'll fire off after a long sequence) and she doesn't really one-shot unlike Silverfist or Queen of Filth. There's a specific rhythm and pattern to her fight that, once you see it, you can kinda just run circles around her.

...Or just pick monk, put overabundance on tempest bell, and bonk them to death.

Lol if you like Napuatzi then watch this (fight I had with here a few nights ago): https://youtu.be/ey8cmUPdoKE

Thank you for those that gave some advice on this. It is mainly the end of Arakaali's Sting so will try some of the strategies around this. Outside of this, the fight is fine and nearly killed her a few times only for this to kill me.

Not tried again since this post. Might try again later if possible but there are other stuff I need to sort first. Still feel Arakaali's Sting needs adjusting as, from what i can gather, unless you have specific builds it can be very difficult to dodge and will just kill, even if played well the rest of the fight.

As an update, I tried the Viper fight again today and with the feedback around Arakaali's Sting - mainly moving to a wall at the end of it - and was able to do this.

100% feel it needs tuning though. This took me 10+ attempts to beat. An example as to why this is so annoying and difficult, one of those fights she was barely on the ground to fight or when she was, it was firing projectiles so fighting as melee was difficult - needless to say this was not the successful one.

On top of this, in comparison, it only took 2 attempts to beat the Act 3 Boss. In both Acts 1 and 2, the final Act bosses were certainly the hardest and that feels about right - a final challenge for the area. This is not the case in Act 3, Viper is notably more difficult from me experience.