Let’s file this one under: bad ideas from movies and TV. Although it’s fun to think your characters are so good they don’t need to train, this simply isn’t the case. Any skill you neglect will degrade a bit over time. This isn’t much of an issue if we’re talking about your ability to speak a foreign language, or tinker on your car. In those cases, there are clear thresholds, either you can do it or you can’t, and if you’re above the threshold for what you need to do, you’re good enough.
The same isn’t true of fighting. If your character isn’t practicing, stretching, and otherwise keeping in shape as a martial artist, their ability to fight will be seriously affected when going toe to toe with a foe who has been.
This is related to the weapon maintenance bit from the Your Character’s Weapon is a Character article. Any martial artist needs to be working constantly to stay at peak. It doesn’t matter how good they are, if they’ve been screwing around, and they face off against your villain, they’re going to lose.
If your villain is supposed to be effective, it’s safe to assume they’re spending all their time working towards their goal. If they’ve achieved their goal already, somehow, they need to be working towards a new one. In short, your villain needs to be active. Moving forward, they are a mobile target. If your hero is lazy, then they are constantly playing catch up.
It gets worse if your character is facing off against their henchmen. Outside of a few specific examples, henchmen or soldiers will be spending a chunk of every day training, exercising, and honing their abilities, even if they aren’t actually learning new material.
To keep up with that, your character needs to be training, practicing and exercising, or they will get outmatched fast.
All of this becomes even more immediate when you consider that hand to hand combat is a dynamic thing. While some of the Asian martial arts pride themselves on being built on traditional combat techniques, modern combat is constantly evolving. Counters are being developed to the strikes that worked yesterday, and those counters will be less effective tomorrow, because of a shift in strike preferences and techniques.
If you want your character to participate in combat with professionally trained combatants, they need to train and be ready to encounter new things every day. There’s no place on the battlefield for a hero who rests on their laurels of being “just that damn good.”