Pin them down with throw-away units such as peasants, then pick them off at your discretion. As mentioned above, once these chariots are bogged-down, they will literally fall to pieces in no time whatsoever. This is because of a lack of mount armour; even though the crews are heavily armoured, this counts for nothing if the horse (or whatever the mount is) has limited armour and hitpoints. Vanilla RTW neglects many areas such as this.