If Reshiram & Charizard GX has the extra 3 Fire Energy for "Double Blaze GX's" bonus effect and attacks a Whimsicott GX with "Fluffy Cotton" Ability, does the Whimsicott GX get to flip for "Fluffy Cotton" or does the "Double Blaze GX" attack go through automatically?

Technically speaking you still flip for Fluffy Cotton, but regardless of the flip's outcome the Double Blaze GX attack ignores the effect and does the damage anyway.

Relevant cards

SM10_140

SM10_194

SM10_20

SM10_206

SM10_217

SM10_226