Move next to the save point near the first Test Of Valor warp point. From there 
you can drop down to the subway. It is here that you can get the 100-hit combo 
with almost no effort. Once you have dropped down, run toward the subway cars. 
Try to get into the little hole under the cars, and kill almost all enemies. 
Eventually you will see two enemies appear near the opening from where you came 
from. If everyone is dead, one will rush up and the other will try to hit you 
with arrows. Wait until the enemy running toward you is almost in the pit. Once 
he is that close, jump up in the air and hit him with shurikens. You should get 
about four hits each time. The ninja will just stand there and not attempt to 
block. Do this as rapidly as possible. As you hit the ninja he will stumble 
back. Eventually you will need to close the distance. Once this has occurred, 
you must start jumping back and forth to avoid the ninja shooting the arrows, 
and continue your shuriken attack on the first ninja. He will die once you have 
hit him about 70 times. Once he dies, you will hear him grunt. You will then 
have to close the distance with the other ninja. He will also run toward you to 
close the gap as well. Repeat the same trick as above (jump and throw). You will 
get the achievement easily.