The key to this game is preparation. Always be wearing the best gear you can possibly have for boss fights and it also helps to know their weaknesses. I have never had difficulty with any boss in Valheim running solo.

I know there are a variety of weapons to choose from and some of them are just bad-A**. However, I find shields are almost a must and most of the enemies will be dealt with fairly easily using either sword or mace(slash or blunt). The exception to that rule is Trolls. They are weak to piercing weapons like bows and spears. If you can learn to parry well (block right when an enemy is hitting you), you should find most enemies easy to defeat. Use any weapon you like if you can use a shield with it.

Common sense tip: Never fight bosses at night. The spawn rates of enemies will at bare minimum have you annoyed and you don't need that during a boss fight.

The Elder is weak to fire.

The first boss that could be difficult is Bonemass imo. If you want to survive this fight, poison resist mead is a must. Also, you should be eating good mid-tier food (Sausages, turnip stew, etc.) Lastly, I recommend having a maxed out Iron mace/armor (personally I use maxed out Troll armor until I'm ready for the mountain biome). Bonemass is weak to blunt weapons. If you have all these, bonemass shouldn't be a problem.

Moder on the other hand can be more difficult. I recommend eating Serpent Stew, Lox meat pies, and either blood pudding (for more hp) or Fish wraps (for more stamina). Personally I use the fish wraps because running out of stamina has led to my death more often than relying on HP alone. I recommend you have maxed out wolf armor/silver weapons and a maxed out Draugr fang bow. If you don't have the recipe for this, you probably haven't harvested muck yet.(It can be acquired by using an axe on the bright green bubbles/spots on trees in the swamp biome). Always use frost resist potion on top of your armors. Dig trenches or build shelters before the fight in case things get hairy and you need to recuperate. Most will tell you to use "fire" or "poison" arrows for this fight but I found using a stack of Obsidian or Needle arrows may work just as well. Lastly but this should be obvious, bring healing meads! If you get hit, you will be using some but if played smartly, most of moders attacks can be avoided.

For Yagluth.... eat the best foods you have and wear your best gear/weapons, use healing meads, and be smart and use Bonemasses ability for damage reduction and you should survive the fight.

WARNING!!! Fulins will start spawning in the meadows biome after defeating Yagluth! If you're like me and hate the random attack events (I wish there was on option to just turn them off....to each their own) To avoid this, I recommend using an alternate/or friends seed to defeat him if you don't want Fulins at your door on the daily.