Im assuming you mean if they raise their defense really high? Cause Barrier doesnt do that in this game, it just makes a barrier that acts like a breakable wall the opponent has to get to. They can also only use that twice in a scene.

The most amount defense can ever be raised is six combat stages, which is 2.5 times higher than their stat. The maximum level for a defensive stat is 30, or more appropriately, the max evasion bonus from any stat is +6, one for every 5 of a defensive stat. If you have a 30 (max) defense and raise it by 6 combat stages, your defense goes up 2.5 times, which is 75 defense, or 15 evasion bonus. That gets added to the AC of the move being used.

There is a rule that is a one damage minimum, but like I said, for myself, I don't use it because I dont like it. If you are in a situation where you'd be reduced to that, it's a lost cause. Dont waste time trying to widdle away an opponent who, at that level, likely has 90 or 100 hp, one at a time while its still beating on you.