Reversals
Maybe you misuderstood? I would think that it would probably take far more than 100 repetitions, but not all at one time! I frequently reccommended repeated writing with tactile input while articulating the letter sound. For example: After dinner smear a little extra ketchup on the plate and let Johnny write the letter while saying the sound. Make a game of it -- write it big, write it little, say it loudly, say it softly, say it in a monster voice, etc. (Frequent finger licking is encouraged!) Do five or six repetitions -- depending on how much fun you're having. You can also use chocolate syrup, mashed potatoes, shaving cream in the bathtub (depending on the age of the child), sidewalk chalk outside, etc.
Eventually, you get enough practice to learn it, but not 100 times at once and not with pencil and paper. Also, saying the sound and/or letter name is important for associating the sound and symbol.