By itself Dynamic Punch seriously sucks and is not worth the 50% accuracy.
But I LOVE it on my Poliwrath, it is just deadly, because my Poliwrath's moveset include Mind Reader + Dynamic Punch, you cannot think of how much times that kills.
Yer just unlucky mate. So am I, Dynamic Punch would miss four times out of 5, and the enemy would hit four times out of five. I have horrible, horrible luck.
Dynamic Punch has too little accuracy to be useful. It seems like attacks such as Sheer Cold and Fissure never work, but they have 30% accuracy, so they become pointless to have.