I tried the method listed by A47A47 and couldn't get it right for the Avatar Achievements so I came up with my own method which is very easy.
1. Go in to Training Mode
2. Choose your Character as Mei , choose your opponent as Shizuka
3. Choose your Style as Quick type , choose your opponent style as quick type.
4. Choose any stage you want
5. Move your character in to the corner with your opponent
6. Push Start and go in to Training Options.
7. Change the following options
- Burst Gauge - Infinity
- Tension Gauge - Infinity
- Guard - Off
- Recovery - Off
8. Return to the game
9. Hold LT ( Left trigger ), after the move connects hold down (2) and keep pushing Y.
It might take a few attempts as seen in the video. By just holding LT you will unlock the first 2 Avatar achievements. You need to do the duck and Hit with Y for the 1 extra hit to make it 50 hits.