Rocketbarrel Barrage (バレルジェットパニック Bareru Jetto Panikku Barrel Jet Panic) is Diddy Kong's Final Smash. When activated, Diddy will be able to fly around with his Rocketbarrel Pack and fire extremely powerful shots from his Peanut Popguns. Each peanut does about 28% damage, while making contact with the Rocketbarrel itself causes 18% damage and has more knockback than the peanuts.
While using the Final Smash, Diddy only shoots downwards and falls very slowly. Moving around causes Diddy to angle himself, allowing different shooting angles depending on his flight path; however this usually amounts to Diddy moving and shooting in opposite directions. Therefore, it is difficult to keep an opponent in Diddy's attack range for very long.
When the attack ends, Diddy's Rocketbarrel Pack violently explodes, sending the Rocketbarrels flying in different directions. This does not deal damage to Diddy. Diddy is flung into the air and becomes helpless. Some peanuts left over from the attack can be consumed for a small amount of health recovery. Sakurai suggested not to fire randomly because opponents can also eat any peanuts that miss and heal themselves, although the peanuts heal very little damage compared to the damage they deal. Players need to take caution to return to the stage when the Final Smash ends, as Diddy can easily self-destruct or be edgehogged while becoming helpless. Fire at opponents hanging from a ledge at a small distance repeatedly, so they can't edge-hog Diddy or abuse the temporary ledge invincibility.
 Description from the Instruction Booklet
Strap on the rocketbarrels and take to the sky. While flying around, you can use your peanut popguns to blast everyone you see.
Diddy Kong's Final Smash. Equipping his rocketbarrels and dual peanut popguns, Diddy can fly around attacking opponents in rapid-fire fashion. He can tilt the barrel jets left or right to control his flight and shoot peanuts straight down, too. This is a technique that tests one's ability to control both movement and attack direction.
Rocketbarrel Barrage comes from Donkey Kong 64 as an exclusive move for Diddy Kong. When Diddy Kong jumps into a barrel that has his face on it, he bursts out of the barrel with a barrel jet pack, and two peanut popguns. Diddy Kong could fly around the area, and shoot at enemies with his peanut popguns. However, this special ability runs off of Crystal Coconuts, and when the Crystal Coconuts run out, Diddy Kong's special ability disappears. Diddy Kong also uses this move to help defeat King K. Rool by shooting at spotlights to make them fall on him.