I've played this game 90% solo or more solo and it's definitely doable but with friends it's more fun and easier to gather/grind materials (especially for big buildings is exhausting). Exploring with friends is easier and most bosses also, even though so far I beat them solo.

Originally posted by RaleMaximus:

One thing to also consider when playing co-op, is the difficulty of mobs and bosses will increase with the number of players. I found that some bosses (Moder comes to mind) are actually easier solo than with a group of friends.

The amount of hp increase is insignificant compared to the double dps you are doing with 2 players (or more). Or at least I would think so.