Do you mean the "Five headed dragon"? As far as I know, seven headed dragon doesn't exist.
However, the weakness to Five headed Dragon would be his inability to negate anything. Just about any spell, trap, or monster effect that can destroy monsters can take him down. Night Assailant, Man-eater bug, sakuretsu armor. But some cards that can be put in specifically for him are cards like Bottomless Trap hole, Karma cut, and Grave of the Super Ancient Organism. You could use min-guts and have a light monster.
Bottom line is... when you're facing a monster like this, try not to think about taking it on in battle, look for a card effect to take him out.
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A dragon type's weakness is, believe it or not, is dragon type.
Five-Headed Dragon is Fusion Summoned by using any five Dragon-Type monsters as fodder.
no but there is a Five Headed Dragon[Dragon/Fusion/Effect] ATK/5000 DEF/5000
Five-Headed Dragon.
Flying Pokemon...