Askelon Phoenix

“Fire damage” is any damage from a fire source, such as a fire minion (including strikes), a fire site, or a fire magic.

Morgana le Fay casting Lightning Bolt is not fire damage, since Lightning Bolt is an air spell. Apprentice Wizard casting Firebolts is fire damage because Firebolts is a fire magic (and in fact would be 3 separate instances of fire damage). Anyone casting Grapple Shot that sends a minion into the Phoenix will look at the minion making the strike; if it’s a fire minion, it’s fire damage.

If the Phoenix attacks and there are multiple defenders, each defending fire minion that strikes it counts as a separate source of fire damage.