uC-adjustable harmonic oscillator circuit @100W
Before you decide how you are going to drive the ultrasonic element you need to understand what the device model looks like. Then you need to decide whether they are in series or parallel resonance (of the multiple elements).
You should start reading here and here, to gain this understanding.

It will be difficult to drive the elements without either a series resistor or inductor as the amplifer damping will lower the Q of the output circuit. 
This commercial offering shows the output drive configuration:



From here on page 112

At 1-5W range you could get by with a series R, but at 100W you will certainly need a series inductor. You don't need the main transformer, but you certainly need a series element and the adjusting capacitors for your task.