Love the people who obviously play on easiest and just say, "use your OP hero". When playing on realistic you get 2 shotted so unless you want to watch the rest of the battle under crappy AI this isn't an option.
You should easily be able to have 2:1 ratio of archers to their HA, place archers ahead of infantry (if behind infantry they wont fire) slightly favoring right side, as HA usually come to the left. Often times the HA group will retreat if they suffer a few losses before they can fire more than a few volleys. Place your Cav behind your army and charge if the HA goes behind you, make sure to monitor them, some might go for the enemy main army instead. At this point if you feel confident in the size and your HP, you can try and chase some down, but be aware that the main force of the enemy should be closing in soon if they are on offense. At this point i often pull my cav back and charge the front line once both infantry clash. The HA should be dimished enough to focus your Cav elsewhere, the last thing you want is your CAV tied up chasing stragglers.