A few people said this already but it needs to be said again. It's all about preparation. You run in to tackle the Bonemass or Moder with the wrong gear and without getting ready for the fight? You're gonna have a bad time. Prepare properly and you'll demolish them. It's 100% on you though, and some people don't get that. They complain that it's too hard but they weren't ready, didn't get the right gear or consumables, and didn't do the proper preparation (and I don't mean using cheesy methods to win).

Not all bosses are difficult. No, really. Some of them are easy. Once you understand how they attack both The Elder and Eikthyr are relatively easy even with very low tier equipment and without any preparation. I've killed Eikthyr with no armour, a shield, and a wooden club and I'm not that good at the game. If I can win with no armour, and the lowest tier weapon and shield in the game then you can definitely win if you've got your gear upgraded as far as possible and play carefully.

Make sure you aren't using absolute garbage equipment (upgrade, upgrade, upgrade), and perhaps do a little more fighting before you fight the bosses if you're struggling. If you fight more (and don't die) your character's skills will increase which gives you a better chance to win against a boss. You aren't on a timer, there's no big rush to summon and kill a boss. Slow down if you're finding it difficult. Take time to get better at fighting, and I mean you - not your character's skills. You. If you gain more experience, more skill, you'll quickly find things get much easier.

Eat decent food. Get the right balance of health and stamina for your play style. I find more stamina is usually better. I'd rather be able to quickly move and not get hit at all instead of maybe having about 20 to 30 more health that's going to *MAYBE* let me take one more hit before I get killed.

If you had to travel to get to the boss (which is likely) do yourself a favour and make small camp a safe distance, but close, to the spot you plan to spawn the boss. Set your spawn point there so if you die you don't have to travel a long distance to get your gear back. Even better - set up teleporters so if you find it too tough you can go back to your main base and gear up even more.

Prepare, practice, persevere, pulverize...

Edit: This is worth a look for people complaining about the bosses being difficult now. They were easier, and they're still not all that tough even with the buffs.

https://www.pcgamer.com/valheim-players-are-killing-bosses-too-quickly-so-todays-patch-makes-them-tougher/