After getting the Electric Shuriken you're on the rooftops with
enemies around. After the first set (where you get the Shinobi
Moji) you swing across two poles to get to four bazooka-toting
baddies. We'll use these, and their friends above, to get the
achievement.
Jump straight up onto the roof and run past the enemies. Some
more clamber up ahead of you. Lure them back towards the bazooka
guys and begin fighting them. Once you have a good few hits (the
dual blades will help here) you can drop down and kill the
bazooka guys. Remember to use Ninja Vision to slow things down if
when moving between enemies, or you get hit.
Alternatively, before you get near to the boss at the end of the
level you drop down and there is a wall of fire blocking your
path. I scored over 50 hits in a combo by using the cyclone
shuriken and twin blades as there are many bats and regular
enemies here.