I'm soloing a lot and currently use the following rings:
Bandage - Healing Ring (Diagnose works too if you don't wanna target yourself, lower heal though)
Slash - Attack (Multiple targets if Raged)
Hack - Attack
Divinity - Stamina Regen
Coyote Spirit - Faster run speed
Rock Armor - Damage Reduction
Pot Lid - Damage Reduction, Deflection (Chance to block attacks)
Improbablilty Sphere - Damage Reduction, Reflection (Chance to reflect attacks back to attacker)
Tons of Damage Reduction means you take less damage and live longer with less healing.
With this setup I rarely have to heal and can take on most bosses solo. Like the Feathered Coatl, Duneslam and the Colosseum Survival on Hard Difficulty.
Remember, you can put on all other buffs as well before you leave the Null Chamber, they usually don't run out until very late in a boss fight.