Been playing it solo and I just defeated Modor (the mountain zone boss, essentially the 4th stage of the game) without deaths. I think the first boss fight isn't anything special solo but for all the ones after the first, you're going to need to be creative and exploit every tactic available to you. Doable, yes, but not without some unconventional strategies. Mostly I mean modifying the battleground with trenches and earthen towers ahead of time and waiting until you have all the best and upgraded gear currently available to you including food and meads.