Durant is kind of a situational card. I sometimes (usually against Blastoise) use him for the first couple turns to set up a second Klinklang by reusing Heavy Ball/Rare Candy/Skyla. But he is mostly for late game, especially when hammers gets rid of most of your energy.
Hitting for 100 every turn isn't bad by any means, but Durant is really good to have for those moments when you really need it.
I've actually never played mirror at tournaments, but in testing, attacking with PlasmaKlang and Cobalion NVI are the best choices late game since they still have to take 2 turns to KO them, take the same amount of energy as Cobalion EX to attack, and give up one less prize. You have to be careful about timing your Durant and Max Potions though, because they'll keep swinging for 80-100 just as often as you are. Hypnotoxic Laser is helpful against mirror as well.
Another thought, how many HTL should be played in this deck? Also, what Ace Spec. do you find the most helpful?
I currently run 1 HTL and Dowsing Machine. I like Dowsing Machine over Computer Search because most of the time, my resources are discarded anyway.