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If, during your declare attackers step, Ursine Monstrosity is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under your control continuously since your turn began (and doesn't have haste), then it doesn't attack

Gyarados' Energy cost is also a huge problem

It can attack all monsters once each, and when it manages to destroy a monster you can set a Trap Card from the GY

And lose a life

Firewall Draagon was known for allowing some of the most frustrating first turn knockout loops in the game's history , as it could cycle monsters almost infinitely as long as they were sent to the GY

A fitting mon for the top spot on this list.