Try wrist rotations.

hold your hands out to your side at shoulder level, make fists, turn at the wrist only and rotate the fist clockwise, go say, 10-15 times clockwise, then do the same counterclokwise.

Open and close the hand very fast. fully open, fully into a fist, see if you can do 30 in 30 seconds.

another thing you can do, is make a dragon hand form and press down, then raise the hand up and make a cranes beak hand form.

do this continually. same with the wrist rotations.

Hitting a bag will teach you more about how to align properly than it will strengthen your wrist itself.

another thing your could try is wrist curls with weights, or the stone on a rope wrapped onto a stick also works for this, reel the stone out, rell it back in.

one more, pick up a staff, about eyebrow height, preferably rattan or heavier. grab the staff at the very buttend so the end is in the heel of your hand. Hold the staff straight out at arms length and hold til failure. do the other hand. Over time, your wrists and forearms will grow stronger and harder. Do you sets to, hit the bag even, but it all works together.

cheers