Just to add to the above solution
The best place to get this achievement is the Fight Club. You want to choose a mid level club. I thought Southwark was ideal at Level 5. Any lower and you will probably kill them in one or two hits so it will be difficult.
Those used to playing Assassins Creed Series will understand the Multi-Kill process. It is very very simple, much easier to do than to explain. Here is my attempt to explain:
Wait until you are surrounded by at least two enemies, attack one enemy until his health starts to flash, at this point pull away and start attacking a different enemy. Keep attacking this enemy. Now both enemies should have red flashing health, at this point press the attack button but while you are killing the enemy, immediately push the analogue stick in the direction of the other enemy and press the attack button again. This means when you have finished killing the enemy you will immediatly kill the enemy in the direction you are pointing.
This is the easiest type of multi-kill, its best to practice the technique but once you learn it you can easily perform triple and quadruple multi-kills which give great animations. The key is getting the enemies health so its flashing. You can technically get 10+ enemies down to flashing health then go on a monumental multi-finisher
Hopefully this explains the mechanics and will help you earn the achievement.