There are only five well-documented fatal lightning strikes on giraffes between 1996 and 2010. But due to the population of the species being just 140,000 during this time, it makes for about 0.003 lightning deaths per thousand giraffes each year. This is 30 times the equivalent fatality rate for humans.
Pic by Luca Galuzzi
But that's the thing, you're NOT out there with your catapult, so the statistics still hold true. Assuming you were given the catapult and I was given a giant tesla coil, I believe I would still be able to zap 30x more giraffes than you could hit with your pathetic catapult.