If you've recently lost a fight against another player who defended with a 100,000% defense boost, this means that you've just been Counter Attacked!
The 100,000% boost is a backend number designed to make sure Counter Attacks hurt even the strongest opponents! In many cases, a low-level player getting a 100% boost in defense still wouldn't be powerful enough to stave off an attack from a high-level player.
You'll only ever see this boost when you trigger an Counter Attack that was set against you.