If you take a 'model' polybian army in the demo it seems very hard to use. I've laregly been using it against a Gallic warband army. That should historically give you a tough battle, and indeed it does. I've found the only way to win is to trmpt them with velites and then wait for them to attack. If you attack with hastati then principes you die, since the hastati rout too quick. If you wait for therir charge you stand a bit more of a hope.

Treating each group of units like a leigon and not as maniples seems to work better. Go for a quincunx and they rout right of the field, deploy in 3 lines and things work better.