Both Gutripper and Doshia can be done in 1 try with no dieing. I did it as 1 level higher Sorcerer, wearing cloth armor 2-4 levels below my level. Used 3-5 healing potions.
It was challenging, yes but it is also doable. Don't stand when Gutripper charges, try to use roots, stuns if you have any, try to avoid him by getting on/off the platform. Go behind pillars. Basically kite him.
Same with Doshia, but also have to kill the healing orbs she gets. I didn't even know those can heal me too. I just killed them before they reached her.
Both these mini bosses are good practice for how you need to play later on in group dungeons. Standing toe-to toe with mobs doesn't work well in the game.
The 3 resistance bosses - are those the Veiled Keep ones? I didn't do that one alone, I had a friend with me and we killed them easy enough. First 2 attacked an then the third after the 2 were dead. Was very easy with 2 players, can't speak about solo difficulty.