How to survive Saladin?

So how do you actually survive Saladin's attack against Jerusalem as Kong Baodouin IV? I mean, I have no time to acquire enough money to hire mercenaries and even so mercenaries and holy orders won't do any miracles, even if you ally the Duke of Antioch. Also, I tried allying Byzantium through marriage but the emperor dies and the alliance dissolves. Seems almost impossible to defeat this guy.