Ceruledge’s Flash Fire makes it immune to Fire-type moves, and if it gets hit by one, its Fire-type attacks get powered up afterward. It does not turn the incoming move into damage, and it does not boost Speed.

Spread moves

Yes, it works on spread Fire moves too. If a spread move like Heat Wave or any other Fire-type attack would hit Ceruledge, Flash Fire still blocks the Fire damage and triggers the ability’s boost the same way.

Important caveat

That only applies to the Fire-type part of the move. If a move has other effects or non-Fire damage sources, Flash Fire doesn’t help against those.

Practical takeaway

  • Fire move hits Ceruledge: no damage from the Fire-type attack.
  • Ceruledge then gets a Fire-power boost.
  • Spread Fire moves are treated the same way.

In short: yes, Flash Fire works on spread Fire moves.