(Psi)SeveredHead
Adventurer
Auras (what makes a good solo aura? how to use effectively in encounter?)
Also how to keep them from killing PCs too easily. When a PC falls unconscious in an aura, it's game over. Often literally.
Save...and then die powers (making "save ends" powers more suspenseful & increasing player agency...you don't just make a saving throw vs. fire, you leap into a waterfall!)
Save isn't a good mechanic here. I use a modified version of the MV black dragon's vulnerability ability. In this case you are affected by ongoing fire damage until the end of the encounter. There is no save, but if you take a specific action (making a skill check, falling prone, jumping through a waterfall, etc) you can end the effect.