You could come back later when you are a higher level with better equipment.
For warriors, if you drink a shadow potion and use the spin attack near (but not directly on) a creature, you will hit it without turning visible.
The Polymorph spell works well on him. Blind then Hell Spikes is effective, as should be survivor's Curse plus any elemental attack. Scorpion traps (survivor's Deadly Gift) are quite overpowered, and would probably make short work of the dragon rider.
Pretty much anything that will work for Josephine will work against other opponents;
Chasing Josie... techniques for defeating Josephine (Spoilers)
