Head hurting yet?
After reading "How Wars Are Won" by Bevin Alexander, I had several ideas, some of them insane, some of them not. This is one of the insane ones.
Step 1.
Buy at least 8 of the best phalanxes you can afford, and buy at least 8 of the worst non-missile cavalry you can field. Currently my experiments have involved 8 phalanxes and 12 cavalry, but I think the mix can be changed. Upgrade your phalanxes with any money leftover.
Step 2.
Put the phalanxes into a formation four units wide and two or three units deep, depending on the army composition. Then put equal numbers of crap cavalry on each flank. Group the central phalanxes. Your choice as to whether you want to group the flanks.
CCC PPPP CCC
CCC PPPP CCC
Step 3.
Advance your central phalanxes as one against the enemy center. If he has onagers, take them out of phalanx and run. If he doesn't, just move in phalanx. Keep your cav where they are. Their purpose is not to threaten, but to NOT THREATEN.
Step 4. If he refuses to move, take your phalanxes out of guard mode, and then move the whole group behind the enemy line. They should immediately slaughter the unit in front of them. Keep your eyes on them and your hands near the Guard mode buttonl; once the enemy charges, turn on Guard mode and use your rear rank to deflect enemy attacks. Step 5. Alone, your units inevitably lose, but they aren't alone. Remember those crap cavalry? Now it's the time they show their, erm, worth. Bring them forth and charge into the backs of soldiers who are already engaged with your phalanxes. Even Round Shields will cause white flags to pop up all over the enemy's puny Urban cohorts if they are being engaged with Sacred Band from the front and 6 Round Shields from behind! Avoid any enemy that turns, you want to hit the units that will rout.
Get the enemy to surround you.
Bog him down. As long as you do not rout quickly, you can drag him to a prolonged fight. Try and get the entire enemy army to attack you, and preferably completely surround your phalanxes.
This tactic goes against the two sacred rules of R:TW - never get surrounded, and never use crap cavalry. Amazingly, however, opponents seem to have a tendency to fall for the attractive bait of eight easily outflanked phalanxes, and like to ignore light cavalry, no matter in what numbers.