How to fight a pike phalanx without another phalanx? Help!!!

[Apázlinemjó]

It depends, what kind of phalanx you are facing. For example if they are levies, then you don't need to pin them that much, well timed heavy cavalry charges from the rear can rout them pretty fast if they don't have morale bonuses (from nearby enemy general and such).
If they are medium ones, well pin them down and encircle the formation with cheap skirmishers and watch the show.
If they are elites, you will need a good unit which can withstand the pikes for enough time till your heavy units eat the elites slowly.

A good tactic against "rotating" phalanx formation is to withdraw with the heavy units if they "aggro'ed" the enemy and wait till the "pinners" take the aggro back.

Edit: Ah well, Frontline and Mikhail were faster than me, lol.