Yeah actually I do have some advice that helped me.
Besides practicing it more...
Wrist strengthening exercises are important! To achieve stronger wrists do wrist pushups on the back of your wrist, alternate each hand when you do them. Do not do a ton of them though a day maybe 30 max.
This guy has the best tutorial in my opinion for walking turtles/darkhammers. This is the video that helped me
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As for jackhammers, you really gotta make sure you have crickets and the pumping motion down fluently. It's going to take time, my advice is make sure you don't stab too high as it will throw your balance off. Stab lower.