Played a few times before the Mistlands update, managed to kill the bosses solo twice now. My experience taught me these rules:

1. Rule #1 of Valheim: You must, must, MUST, keep up with armour, food and weapon requirements for each respective biome. If you do not, your life will be FULL OF PAIN AND SUFFERING. The game is absolutely unforgiving in that regard. Especially when you face 2-star opponents.

2. Build your base on a semi-island, where most of the landmass is surrounded by water. Seal off the base enterance with a moat, and make sure the enemies can't swim around and flank you either. A water-surrounded elevated landmass works best, this will protect you from most mobs except for drake events ('A cold wind blows from the mountains...'). Be sure to spread enough worktables to cover the whole landmass to prevent enemies from spawning.

3. Experiment with new stuff: Discover a new ore? See what you can do with it. Discover a new plant? Find some seeds? Plant them, harvest them and see what recipes are unlocked. These discoveries may be vital in keeping up with Rule #1.

4. Experiments with mods. Despite the game still being in Early Access, there is a significant number of mods for it (Eg. NexusMods). These can alter your gaming experience significantly, starting with new building objects and finishing with game mechanics configuration (Eg. There is a planting mod that allows you to plant most mushrooms/plants/trees).

Have fun. 😀