NINJA BLADE

This can be obtained during the chase at the start of the level just shoot the big bats with the turret and when you are reloading press LB to activate your Ninja Vision keeping the Combo alive if you do not get it here do the same thing when the train comes from behind you.
The Achievement pops at the end of the level

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.

Just use your Blades, you'll surely get over 30!
Use it when you see lots of enemies, or the area with bats