i have been fiddling with this for a while.

flank support is the way to go (as it should be)

example:

i put a unit of argrypisdai (or whichever is the uber heavy seleukid phalanx) and i controlled a unit of principes, and a unit of cheap celtic swordsmen.

principes engage and immediately lose 10 men, and are losing men fast. by the time my celts have circled around, my principes are on the verge of breaking.

my celts (again, cheap, unarmoured i believe) charge, and the battle is slowed.

i end up winning the day.