Its 100% doable solo. I am super bad at soulslike games, but this one is much easier as its mainly ranged focused. As bosses scale based on the number of players (hit points, damage, sometimes number of add) some of them are even easier solo and the game is more predictable and tactical.
Its true there are some annoying places where coop is an advantage, mainly where you need to defend something or someone from waves of enemies, but that is probably 2% of the whole experience and you can always re-roll worlds to get a different boss and dungeons.
There are also summoner/turret/sniper builds to even the odds, but you can pretty much beat it with any build.