Brock caught Forretress as a Pineco near Azalea Town with one of Kurt's Fast Balls while trying to pick Apricorns.
The day Team Rocket sent out their 'Plucky 10 Times Over' Robo Spinda to capture all the Spinda and
Claire's "Lucky Spinda",
Brock's Forretress was sent out to battle against it.
Brock knew he needed a powerful attack as his
Brock's Mudkip and his
Brock's Lotad weren't even able to scratch the surface of the mecha. Knowing that his Forretress would suffer,
Brock told it to use Explosion on the mecha which proved to be very effective as it was powerful enough to destroy the mecha.