Other answers covered points, but I saw nothing on a little bit of equipment for your NPC's. You would have to assume that any NPC's on the outlands are used to engaging in fights on the open terrain, and therefore would come equipped for it.
Bolo and other immobilizing weapons (nets?) Hard to run away when you can't move. Forces the other PC's to come in and help.
Spears and javelins In more 'historical' terms, skirmish tactics were often employed vs archers...they may not have the range archers do, but they can still pack a punch. Consider sundering at this level...breaking an archers bow is as good as a kill in war terms and it was a very readily used tactic vs archers (little mean in a DnD sense, but one or two sundered weapons will force PC's to new tactics until they can re-equip). Weapons such as 'pilum' were designed to bend after being thrown to hinder movement around them as well.
Horses and other mounts Mounted units that possess charge would quickly tear through a group of dedicated archers no matter how fast they run around.
tower shield.
Who needs cover when you carry your own?
And a couple tactics the NPC can resort to:
Lay down. NPC lays down, archer receives minus to strike a prone target, assuming they can see even see them. Bonus points to NPC rogues that can potentially turn these into surprise attacks.
Run them into a surprise. two NPC fighters moving slowly towards them and pushing them back into two NPC rogues just waiting for the PC's to be forced back into them
Grapple. If there is an opportunity to tackle an archer, take it!
ANything else I can include is already in other answers.
Edit:
Flank and pincer style tactics should challenge your PC's a bit as well. Dedicate one (or more) NPC's to running up along side of your PC's on their flanks with the ultimate goal of getting behind your PC's while the slower NPC's progress up the middle. Any NPC that is engaging people on the badlands should be quite aware that they'll need to encircle their opponent in order to catch them.