This game is fun both in Solo and Multiplayer and with the mods that are available it really shines!
I have a solo world i play on as well as a server for a few friends and solo you just need to prepare properly. Go to the black forest but upgrade your equipment first. Fight the second boss but use fire arrows, kite it round and watch for roots etc. If going to the swamp take healing potions and poison resist potions etc etc...

Its not easy mode and it is a challenge in places but it is fun and achievable while playing solo.