Originally Posted by TinCow
Shower them with missiles of any kind. Cavalry against this is an absolute no-no unless they are very low on morale and/or you don't mind losing the unit. Otherwise, charge with as many heavy infantry as you can from as many directions as possible. Fortunately, these formations are only usually encountered when the spear unit is isolated, so you can often take you time and surround them fully before they charge. This formation takes some time to get into as well, so a hasty charge when you see it begin to form can disrupt it.