It all depends on luck. If he faces cavalry at the beginning of the battle, then yes, it's over for the poor guys. But if he attacks first and doesn't target them, but instead attacks Hydras (so there's no one else in range except Hydras), or if only small units are opposite him, units he can't reach, then the Paladins have a great chance of not taking out anyone at all. Because all you need to do is place some Lizards next to him (just place them, don't attack) and have everyone attack him.

Added after 8 minutes
Version 3.0 drastically changed everything in the game. Now, on long maps, I play as Iris, and on short maps (of course), I play as Vittorio. What a twist! Before, only the Stronghold could create a rapid-fire ballista with fire arrows, but now it seems like everyone can. I haven't seen two heroes clash early in the game with such ballistas, but I assume that both armies will suffer huge losses, and the winner will probably not be able to storm a Castle. So, by giving everyone a rushing ballista, the creators have effectively buried the possibility of rushing altogether.