This is why a small, but significant force of cavalry on one flank — not both flanks! — wins battles against the Fatimids: a column of knights is more than forty times stronger than a column of light infantry (provided they're the same in numbers and technology), based on the morale, defense and attack ratings. The flanking column can only be attacked at great costs, thus weakening the enemy in the center (where all their units are, including the Mameluke super-unit). The center columns face off against each other normally, but yours gets the flanking bonus. This can make up for tech and numerical deficits, as long as your number of knights is about 10% of the enemy's total army size (a guesstimate), and they're all stacked in the center column, which the Fatimids are prone to do.
Archers are good in theory vs. Fatimids because every battle opens with a skirmish, and the Muslim levies will be decimated from the get-go, but they suck massively against horse archers and heavy cavalry (read: Mamelukes). Really, you only need the archers because the Fatimids have so many, also courtesy of the Mamelukes, otherwise your infantry croaks too fast for your center column to hold the line while the flanking knights whittle the enemy down.
I repeat, heavy cavalry is the best. You need them. Light cavalry is usually good, but not against horse archers. You need those for strengthening the flank, and most importantly the post-battle mop-up phase. Archers are important because the enemy has tons of them. Pikes are mostly not so hot, because archers and horse archers ignore them. Light infantry are the meatshields. Heavy infantry are the backbone, but they're useless in skirmish.
EDIT: Light cavalry in a column without heavy cavalry can actually receive huge bonuses during skirmish (we're talking up to 300 percent). The trouble I've had with pure light-cav columns vs. the Fatimids is simply that horse archers ravage light cavalry, and the Fatimids have quite a lot of horse archers. Similarly, pure heavy-cav columns are usually routed too early, since they're rare and expensive units and you won't have a lot of them sitting around. That's why I'm usually forced to mix my cavalry stacks against the Fatimids.
