I've rinsed a few of the side quests - the "Finders, Keepers" one on the second location I've done twice (different rewards - Raging Bull equipment) and the "Wreathed in Flame" on the first location. Both reasonably easy to solo but as we worked out last week if you want to do a full quest together then the Side Missions are better. Otherwise bring someone in via a cup for the boss or a tricky bit. "Finders Keepers" you can do whilst avoiding the one eyed oni if needed too, and when you complete it give the level a good run around as all the mobs will be dead and you can pick up chests and loot locations.
The second side mission on location two I remember being tough - and I think that is where I quit last time - so good one to team up on. "Kanbei and the Overlord".