First, try removing the 100ohm series resistor. A piezo element is roughly like a capacitor electrically, so you don't have to worry about current flowing through it. All the resistor does is slow down the current charging the piezo element, which means it moves slower, thus producing less volume.
If that doesn't do the job, try the reference circuit in the datasheet:http://www.tdk.co.jp/tefe02/ef532_ps.pdf
An Arduino can only deliver about 40mA of current per pin, so it's not really an audio drive circuit. A good transistor can move a lot more current (the 2N7000 mosfet is rated for 500mA of pulsed current, and the SS8050 bipolar can swing 1.5A aound).
A transistor will also let you use a separate, higher voltage power supply for the piezo element. It's rated for up to 30v, so a 12v wall wart should give you plenty of power.
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