The Dragon Hunting Swordsman (SBC1-ENS09) [Speed Duel: Streets of Battle City]

Once per Duel, you can discard 1 card, then Fusion Summon 1 Fusion Monster that mentions "Buster Blader", using monsters from your hand or field as material. While the only monster you control is "Buster Blader" or a Fusion Monster that mentions it, monsters in your opponent's GY become Dragons.