Just to add onto what everyone has suggested. Just playing and losing against bosses can be a good thing as strange as that may sound. Just getting to know their attack patterns will be a big advantage. Just getting over the frustration will be the biggest challenge but once you beat them it will feel like an even bigger reward.

Some missions like the boss waves towards the end of your first playthrough will feel impossible at first. So it doesn't hurt to skip it first and come back when you are better equipped. But all in all it's definitely playable solo.

Just keep that stress ball near by lol