Firstly, you need to change your starter pokemons attacks to:
1. Toxic
2. Doublekick
3. Fly
4. Anything

Step 1 :
Click 'Trainer Battle' and enter the trainer number: 482301 Then start the battle!

Step 2 :
You should attack the ShinyBlissey's by following these simple instructions:

1. Toxic
2. Toxic
3. Fly
4. Fly
5. Doublekick
6. Fly
7. Fly
8. Doublekick
9. Fly
10. Fly

Just keep repeating steps 5 onwards.

This should defeat the Blissey and not touch your HP.
Keep doing this cheat through all 6 ShinyBlisseys and your Pokemon's level will get higher and you should gain more and more money!