There are several ways to beat this one. However they all center around the same concept, that is for you to get the maximum use of you Priest don't let him get attacked. What does that mean?
The Computers AI automaticlly attacks(at least in this seanario) the first unit that comes into its LOS. What you need to do is move your priest to the far side(left or right) of your attacking forces. Once the ballistica is engage in firing then move your Priest toward it and convert it. The key is that once it destroys your other units it will attact your Priest. So keep it busy. Towers and Walls are great for this.
The most effective use of Priest when tring to convert enemy units is for them not to be in the Line-of-Fire. Always engage the enemy with another unit(yes a decoy). Once you convert the enemy have a refuge for them. In this case that is not necessary since once you convert the Ballista you win.
One last note(maybe I shouldn't tell this).
If you take two Compys and a Priest for every 2 CA you will win.