how, exactly I think he is wondering. OK, I'll give add little more detail to this thread. The scope will be open battlefield destruction of phalanx units, including the tough, high-morale Spartan. First, Sun Tzu actually explained it thousands of years ago. Know your enemy, know yourself. That is, know your units, know the terrain, choose the conditions of battle. Generally, I have another condition for defeating Phalanxes... I want low casualties in my cavalry, which is the backbone of many of my own open-field phalanx attacks.
1. Choose the terrain. You don't want woods for cavalry (I hammer with cavalry, includilng my general(s) usually). I need open area, out of range of towers and walls. If possible, lure or allow them to come to your terrain of choice. Fight to the strength of your unit's terrain abilities.2. Tire them (if possible). Bait/lure, and know that Phalanx tire much more quickly than cavalry, especially if you can get them to drop their phalanx formation temporarily and charge you. Make them come a long way and make them go UPHILL to reach you. Pepper them with arrows, for instance (if enemy Cavalry charges, which I personally hope happens, then destroy the enemy cav, then the Phalnax, but that's another thread topic). Do not let them stand and recover when they reach you.3. Use maneuver! Use your cavalry speed and threat to break their formations... not attacking, but by moving around them and seeing their reaction. Break a large army into segments, separate those enemy chunks that will "answer" your maneuvers, and pick the "weak" or vulnerable chunk (e.g., a group that has turned and now faces/moves downhill), and bring all cav units necessary (use judgement/experience to know how much, and if you will need a general) on it and shatter them.4. The physical assault. It must take place knowing the strength of your cavalry unit(s) and the history of the units your are attacking... what happened to them with arrows raining on them (wavering, e.g.), what is/has been their level of rest (warmed up, winded, tired, exhausted, etc)... This will tell you how quickly they will break, and set your attack vectors and quantities.4.a. Know your cavalry, and know all about the stats, esp. the armor, charge bonus, etc. of the units youi have. Know their state of rest and morale.4.b. Attacking & routing a Spartan Hoplite unit with minimal casualties to equites will likely need some prior assistance from terrain, support units, or location of battle. But if you march them around, weary them (e.g. at least winded), reduce some numbers with slingers or archers (you need to get some rear or side shots with good slingers like balerics or creteans to make progress here), think isolate them and hit with cavalry.4.c. How to hit. It is not enough just to hit a stong phalanx unit (even a rested Levy) from behind to cause rout. You must create a short term, very high casualty rate, and get them to feel surrounded. For tough units like Spartans, it is best to coordinate several strikes in programmed sucession. One or two units hit from the rear, the phalanx begin to raise their weapons and maybe even re-face, then two more cav (or even infantry) make contact at full charge from what was a few seconds ago the front. If you mess this up, which will happen sometimes, well the Cav units will suffer, while the rest of the battlefield begins to close on you (remember you split the enemy apart, but now they will be homing in on you). However, if you do it right, even a Spartan will quickly rout.5. Withdraw certain units: prepare to attack the "next" chunk of enemy phalanx without delay. Do not let your units decide for themselves to pursue on an active battlefield, with formations of unbroken enemy Phalanx milling about. Only you, the human, can assess and maneuver them in prepartaion for a repeat process on the next and the next formations.6. Routing. When the path of retreat of the broken Phalanx and the tactical situation permits, I will continue to kill the retreating enemy. With militia, levy, or plain phalanx, a small, weak cav unit (e.g., lots of casualties alread, and maybe winded/tired) can continue the rout while your otehr units turn to attack the remaining unbroken enemy units. If it is Spartans you pursue, you will likely take casualties if weak cav attack them even when they are routing. Crush, don't nibble, Spartans. You can use infantry to assist.7. Catchers. For open field battles, use 'catcher' cavalry to intercept and ensure minimal enemy flee the field. They can also be used to ensure the routes do not regroup. Normally use weak and/or reduced numbers units (e.g., a 13of54 unit equite can do the job on a fleeing remnant unit, usually). Your maneuver skill, and cavalry lives pay the prices (normally a small price in open battlefields) to get a Phalanx (or any) unit to rout. You don't want to face that unit in later years, since you would have to do it all over again and pay perhaps a higher price. Commit to battlefiled elimination, and this means knowing or at least understanding how units rout, and the paths/ways that they will flee. This will in turn influnce how you maneuver, and which units and in what order you attack large formations.
I have left out many elements which are part of this too, like dealing with the enemy generals, archers, spearmen, etc. as you break enemy formations, for instance. But this is an introduction to open field Phalanx battles (human cavalry ve AI Phalanx). Human vs. human play can require a significant alteration in scope, and but the tactical principles are the same... you need to adjust, as Sun Tsu explined, by knowing your enemy . But that is another topic (H-H vs H-AI tactics). Lastly, to keep the post length "short", I have not touched on urban tactics vs. phalanx. All that is required, however, is to reassess the changed battlefield and the constraints of the opponents.
Happy hunting...