I was having real trouble with dueling dailies, both to kill valiants and later champions. I developed a strategy which has worked perfectly for everything.
I'll be abbreviating button presses with parentheses
1. Stack 3 Defense (4)
2. Talk to your opponent and accept the duel.
2a. Valiants start with two stacks of Defense, and Champions with 3.
3. The opponent will jog to the center of the arena. During this time hit them with a shield breaker (2).
4. Get into melee range and use your lance (1) whenever it is up.
5. Here is the key. Whenever the mob has a stack of defense and your shield breaker is up:
5a. Side strafe to get into range.
5b. Jump and mouse-look turn while hitting the (2) button.
5c. Run back to the mob.
6. Keep repeating 4 and 5 until you kill the opponent.
Using this strategy you should technically never get hit with an attack which will remove a stack of defense. Obviously if you do, put another stack up.
I don't use charge because oftentimes you'll get hit with a shield breaker as a result of having to get far away. The charge's minimum distance is much larger than the shield breaker.
I can basically kill 4 champions or 3 valiants without ever having to use the healing skill (5).