Well let me be the first to point out that one of the things that really helps is using your hoe to flatten the surface surrounding the spawner before you start the fight. This helps alot with staying mobile without dropping into sink holes all the time.

That being said i do agree on the poison side of things, its a massive pain and not very well executed. I've died more to poison effects than anything else, being it direct ticks to death or just ticking down so low some random low level mob gets the lucky last hit, its infuriating, especially with health pots being on a long cooldown.

The boss fight itself isnt great either, I agree summoning adds is a cheap and unimaginative way of increasing "difficulty" in boss fights. Then again i do kinda like his almost immunity to arrows so you cant cheese him by just bowing him but the fight itself isnt very engaging. Run in, smack him with a blunt weapon, block or run away, rinse and repeat. Does make it better than straight up pincushioning him with arrows, worst kind of boss fight tactics imho

Stamina is pretty much a learn to adept thing, you can use food that increases stamina more than health and that helps but the innate wetness of the swamp ruins any kind of regeneration of stamina. Not even the rested bonus is enough to counter it. Its the same with the deer boss causing a thunder storm and leaving you wet and reducing your stamina regen, its a cheap tactic.

There's alot of room in improving boss fights, these days i just skip them as they are annoying AF, cept for Eykthyr, i just smack him to death with an axe as he's too easy to be even called a boss.