Was this directed at me? Sorry, forgot which April girl I was.
If yes then were mercenary clans rebuffed after the infamous debuff? I was on and off Warband recently so I did not play last patches. Last time I played Arboreals were rather pathetic, along with most other mercenary clans and I felt like forest bandits are more dangerous.
As I said before, the easiest and most trivial way to counter AI archers is shieldwall. Just order your infantry in to the shieldwall, make it move towards the archers and then charge them once melee starts. Ironically enough, closer to the archers you are, more effective shieldwall is, because AI always targets torso, so more accurate their fire, less of a chance that stray arrow will hit unshielded parts of the body. There is short stagger when arrow hits and it seems, that if another follows right at that moment, it can pass. But for that you need to have multitude of archers shooting at the same target at the same time.
Cavalry can also be effective but the trick is to split your cavalry in to more groups (2 should generally be enough) and time waves so that one wave attacks as the other one disengages. Try to send them from slightly different angles so that they don't bump in to one another. Point is to have archers always engaged and thus not allowing them time to shoot in to the backs of disengaging riders that are reforming for another charge
Combination of melee cavalry and skirmishing one also works well as skirmishers will circle around archers and confuse their aim, again preventing then shooting at the melee cav effectively.
And don't forget that you can shieldwall cavalry as well. Shieldwall them and then order charge, they will keep their shields up thus reducing casualties during their head on runs.
Of course the most effective way is to use combined force. Make your infantry in a shieldwall attack them frontally, then hit them with cavalry from the flank or rear.