I've played a few gens of 1h/shield. The most difficult opponent to face is what I call a "ninja" (regardless of weapon they use). They are high athletics, high wpf, 2h users. They are very fast, use fast weapons, and they are hard to get the upper hand with footwork. Fortunately, they have low durability.
There is no magic move you can do to defeat them. Every enemy is different. If you try one move (circle right while spamming left attack) it may help you kill an enemy, but it will not help you master these opponents all the time.
Here are the tips I have. There may be some repeats from previous tips.
1. Keep them in front of you. They will try to circle and hit your back/sides in order to negate your shield. Be as persistent and stubborn as a bull dog, keeping them dead in your sights. Since they can easily out-footwork you, this is your best bet. Don't ignore all other aspects of footwork, but focus on this one.
2. Bring the right equipment. This means weapon and shield.
For weapon, bring something long, but not too slow. Since they are fast, if you have a short weapon they will try to play the distance game with you. All the high tier swords do enough damage. I call them "fly swatters". It's always good to have a backup weapon (like an axe for fighting other shields). Consider bringing a fly-swatter for killing ninja types.
For shield, I recommend a strong yet pretty fast shield like heavy kite shield, or an even faster one, but get it loomed if you can. These "ninjas" have lower PS usually, and fast weapons like katana or longsword. If you have a strong shield it will be frustrating for them because they can't break it. It will allow you to take many hits, in case they are spamming the shit out of you. It will also allow you to...
3. Have patience. If the situation allows, don't press the attack. They are the ones pressing, trying to kill as fast as possible. If they are overwhelming you, fall back a bit. Let them hit your shield 3 or 4 times without even trying to attack back. Let them run around you in circles until they are dizzy, then do a quick faint and strike at their head.
4. Aim for the head. Many of these ninja types wear little to no head armor. Make them pay for it.
5. Pay attention to their feints/ non-feints. Some "ninjas" will just spam. If you see them doing this, dont' try to feint of they will just strike you. Some attackers feint once or twice EVERY TIME they attack...if this is the case, just spam and you will hit them while they feint. Just try to notice a pattern, if there is one.
6. Don't just left swing. This goes against some of the previous advice. Yes, the left swing is fast. You know that. THey also know it. They know its coming. THEY KNOW ITS COMING. It's so predictable. I'm going to give you my secret now: overhead strikes. Seriously. It's less predictable, has a confusing 1h animation, is fast, and still goes for the head. They are hard to land sometimes, so practice it. Still, I kill most of my "ninjas" with this attack. It's not as reliably blocked. Also, right swing is slow, but in the correct timing, it can hit them because they will underestimate its reach.
Hope some of this helps! Good luck!