If you REALLY want to keep down losses for the chosens when taking a cav charge, put two units over exactly the same area so one end up on the other. That formation for any barbarian unit - falxmen, swordsmen, axemen, chosen axemen etc. - can stop almost any cavalry charge. That's what I did in an early battle in my Dacian campaign when I wanted to destroy an army of perhaps 10 macedonian light cavalry plus some footmen with my 8 falxmen and 2 cavs, and it worked well.