Here is what I do
- activate native AI and activate horse archer AI
- put all horse archers (with horse-archer AI, so no skirmishers) in a separate group. Place them on the right flank, other cav on the left flank
- let the enemy come near, then unleash your horse archers and advance your normal cav. Use your normal cav to bait the bulk of the enemy horde.
- watch your enemies perish in a hail of arrows.
- careful: when you see horse archers suffering losses, they are out of arrows and need to be pulled out asap.

The cavalry advance with the infantry with native AI? Or she charge like a moron solo?

With native AI, armies form lines and then cirlce around each other until the lines intermingle. The outcome is much better for Rohan than the usual braindead disorganized frontal charge.

Great, I will make the change immediately

This is indeed the best solution, the rohan's losses are reduced to a correct number, as their infantry is nearby to support them. Thank you very much, I can take advantage of this faction ^^