The following setup will basically make your character immortal:

Equip four Eyeball Rings
Rune Slot 1: A Strike-Back Rune
Rune slot 2: Double Edge Sword (If the character has a 2nd Rune slot)
Rune slot 3: Drain Rune (If the character has a 3rd Rune slot)
This setup is not cheap, but it is possible to handle many Bosses solo with it. You may need to remove the Double Edge rune from some characters because they do not have a lot of HP. Go to the big hole near the rotating bridge. In the area before the ruins, the Eyeball Ring is commonly dropped.