My Oracle Ceresa once killed three dragons.
First she summoned some roadblocks (Summon Skeletal Horde and others), then she cast 'Kill' 3x.
Luckily, the battleground was one of the largest in the game, so she could run away while casting. She probably would not have survived a single dragon's breath. Oh, and she had very high spell resistance, so, with some luck, she resisted the 'Fear' effect.
This could also have been a bug, as it seemed she only had to resist once.