Guerilla warfare, 3.5 style: Use the terrain as much as possible. The best possible might be sparsely forested hills or something with plenty of cover to hide or duck in and out but allow easy movement on horseback.
Use 3 deep woods snipers with mounts of their own, invisibility, spider climb and excellent hide abilities. Add in a sorcerer/wizard to add some ranged magic, buffs, and illusions. Have them take Far Shot and have 2 fire from one direction, say to the left and behind the party, that targets their mobility or long range fire-power(casters w/fireball) 1st. They should be far, far out to use the Far Shot without too much penalty to strike at the relatively lightly armored casters or mounts. Have them on the ground and ready to move once the PCs start in their direction. With one(or two) sniper(s) remaining in the tree tops above as spotters for the ground crew, they will be available to strike the PCs once they turn their backs or better, separate. Take a few feats to help with this, like Rapid or Many Shot. If they do separate, do not (as a DM) allow them to triangulate their locations by having them see where the 'hidden' second attacks are coming from. Have the ground snipers keep firing until the treetop spotters signal them that the party is coming towards them. Have them ride on their own mounts perpendicular to the proverbial path the party is on, and if they turn back to face the 2nd attack from the treetop snipers they each quaff a potion of invisibility, and the ground snipers set up to fire again from horseback. Once a mount or PC falls, they will need to start the healing process - then target the cleric while she does this. The cleric will probably get there in time to heal whomever back to full, at which time the attack ceases for a minute, allowing you to lapse out of rounds to give them a sense of security but still keep the buffs. Then repeat the ground attack from a different location until they start chasing again, which is when the treetop snipers should change locations before firing again. Pump up the ground sniper's ride, hide and wilderness skills, and the treetop guys should have excellent move silently and climb (with spider climb) instead of ride.
The whole idea is to avoid melee combat, taking out the abilities of probably half the party. Let the magic-user fling some spells out blindly at a few extra horses (the treetop snipers' original ride) that purposefully cross the path for them to chase and discover the dead decoy horses. Keep the cleric using up spells to heal and make the casters waste spells in the wrong direction. Use alchemists fire as a distractor if the party gets too close to one set of snipers, and if they corner the treetop snipers use tanglefoot to keep them from ascending.
A sixth level sorcerer will be able to send out a fireball, which may be the best 1st strike from max distance. I think Magic missle is a long-range spell, which would be more annoying than anything else, wearing them down or making the cleric use spells. Keeping the party frustrated will lead them to make more mistakes and take bigger chances (such as splitting up or go hunting on her own) which may be a big mistake if they are trying to catch up to the ground snipers on horseback as they keep getting beaten up. The players won't like you when they can get to within 5' of your snipers, but tell them it's a weakness they need to solve.
Think of using the 0-level spells to attact their attention too, like ghost sound, dancing lights or flare. You could use Message once too, just to freak them out. Once you take out a warhorse or two, they will be more handicapped in movement - keep up the sniping by the ground team until the party splits up to chase them, and have the treetop team rain arrows on those left behind.
A good ranged build would be a fighter - each would get 6-7 feats to spend on Point Blank Shot, Precise Shot, Rapid Shot, Far Shot, Weapon Focus & Weapon Specialization, or even boost the reflex save.
For the sorcerer, add Enlarge Spell (Magic Missle, Fireball). Take spells like Mount, Fog Cloud, Silent/Minor Image and Ventriloquism to throw them off, and Protection from Arrows if they have a good ranged guy in the group. You could take web, but for this 7th party it would be more of an annoyance than a threat. Take cat's grace instead to add to her friend's attacks. Definitely use Fireball, as 6d6 of damage in one initial strike when they are unprepared (reduced Reflex save?) should be a nice way to zap the casters and healing fast.
Things to worry about:
The casters might be able to summon some pretty fast critters to chase down your guys, like a hippogriff or elemental. Watch out for them and it may be best to target fire at the summoned monster in order to keep their location safe, but don't hang around long enough for the party to catch up.
A warhorse can run pretty far in a round, so keep a safe distance and accept the penalties associated with the range increments. A negative equal or greater to your BAB would be better than giving up the safety of distance. If one of them is hasted or there is a super-fast monk, you may have to back up even more after the initial fireball.
An arcane caster may be able to take down your mounts in a single fireball blast, so it may be necessary to have a back-up mount (like the 2 for the treetop crew) or separate. If the party is getting close to either set of your guys, have that pair separate and bring in the other set to do damage or draw them off.
Phew. I have been brewing something like this to run in a few sessions, and I hope you have fun with it.
