You can get this achievement very early on in the game and very easily, safe too.
When the Gates of Hell store first opens. Go in and go to techniques. Here you can see techniques to purchase for Bayonetta to use. If you select one, you can try it out. Anyone technique will do, so just select one and go into the practise arena by selecting TRY.
You should be now on a red screen where you can practise the move you want to purchase. This is also an OPEN arena to practice any move you own.
Jump up and just to float down ontop of the enemy. When you are over its head, press A to jump again. Bayonetta will shoot bullets if you are on it's head. Repeat this over and over and eventually it will die (took me 20 tries).
You can use this technique for The Deepest Cut as well.