'
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.