You might try an alliance with the Byzantines; the Armenians even share your religion, although the Seljuks are, at the starting date, invading Armenia already. They seem like your best prospect to actually show up and help against the Fatimids.
Maybe you could go for the If you can’t beat ’em, join ’em strategy and get Shia Muslims to educate your heirs, at which point the Fatimids can’t jihad against you, but you can against your smaller Sunni and Christian neighbors. You might also be able to expand to the south and east until you’re big enough not to get steamrolled, and if the Fatimids get some severe bad luck before taking you out, like a Crusade, a war with the Seljuks, Crusaders or Byzantines, preferably at the same time, you might be able to take advantage of that to dogpile them. Immediately mark the Fatimid Caliph as a character of interest and watch for opportunities. If you can manage to beat them once, sieging the Caliph’s personal holdings and wiping out his retinue, that could seriously destabilize the régime, because the Caliph will have fewer troops than his vassals for a long time to come, emboldening them to revolt. With luck, you might even cause a regency: every battle is a chance for him to die, get captured or suffer a head injury. In the best-case scenario, they might go into a death spiral in which you, Byzantium and Jerusalem can pick up the pieces.