I see these guys as being dangerous because the are a direct reflection of an adventuring party, versatile and self sufficient. I left the stats out, but here is a good composition of forces. I used the core books, plus the five expansions, (Tome and Blood, ect). Levels are subject to your alteration.
Leader, Fighter 2, Anti Paladin 9
Two levels higher than the others, (I chose 9th level for a round 4 feats, 2 stats each), He has greater experience, perhaps even recruiting promising orcs in life, and taking then into death as an initiation. It is even possible that he is a higher level, and that his current contingent are lower levels, but that he has many other recruits available elsewhere. He simply brought these five because he doubted he would need more.
I chose Anti-Paladin because he needs intelligence, cunning, leadership, and raw combat ability. I personally dislike the Blackguard, so an Anti-Paladin would simply be a core class, but exactly the opposite of a Paladin in abilities, (i.e. harming hands, smite good, ect). His aura might even make him immune too turning, while providing a profane bonus to allies vs. turning in that radius. With his mount being larger than the others, it would make sense for it to be a special mount, thus gaining a number of special abilities.
Feats; Mounted Combat, Improved Sunder, Improved Trip, Knockdown, Power attack, +1 of choice. Obviously a lot of ranks in Ride.
Items; +2 Unholy long sword, Equerry's Plate, (+2 full plate, +1 to Ride, mount gains +10' movement and +2 Dex).
Ranger 9
Second big gun, skilled in the tracking of enemies, can act as a second scout to the rogue, (se below), another fighter, and some spell power. It is also possible that his mount is his animal companion.
Feats; Depending if you go ranged or melee, though Mounted Combat is a must.
Wep Focus (melee), Off Hand Parry, Ride By Attack.
or
Wep Focus (bow), Rapid Shot, Mounted Archery.
Favored Enemies would be Humans and Elves.
Items; Rhino Hide Armor, plus either a +2 Keen, Distance bow and Bracers of Archery, or a decent pair of melee weapons, preferably with the wounding quality since he would have the most attacks of anyone. Also max ranks in Ride, plus Spot, Listen, Hide, Move Silently, and Search.
Rouge 9
Scout and back up fighter, he plays off the others and strikes when he can sneak attack, or ties up another Rouge, since he cannot be sneak attacked.
Feats; Quick Draw, Combat Reflexes, Expert Tactician, Flick of the Wrist.
All the sneaky git skills, but UMD is a big one.
Items; +2 Silent, Shadowed chain shirt, +2 Keen, Wounding dagger, (plus 4 +1 Wounding throwing knives OR +1 Keen, Distance bow).
Cleric 9
The backbone of the party more so than any other group, because he can make up for an inherent weakness they all posses. While not one to shy away from combat, he is definitely a support person.
Feats; Extra Turning, Quicken Turning, Heighten Turning, Profane Spell, (as DotF Sacred Spell, but dealing 1/2 negative energy damage instead of divine).
Items; +2 Fearsome full plate, +2 Chaotic mace.
Wizard 9
Also mainly a support person, in and out of combat. He makes most of the groups magic items.
Feats; Enlarge Spell, Energy Substitution (Cold), Scribe Scroll, Craft Arms and Armor, Craft Wand, Craft Wondrous Item. Raven Familiar.
Items; Bracers of armor +3, plus lots of scrolls and several wands.
Sorcerer 5, Elemental Savant (Fire) 4
The closest thing to a berserker the group has, he likes to get dangerously close to combat. He WILL burn everything in sight with his biggest spells if not held back, which he usually isn't.
Feats; Empower Spell, Energy Substitution (Fire), Enlarge Spell, Pyro (or Spell Focus (Evocation)). Stirge Familiar.
Items; Major Ring of Elemental Resistance (Fire), +1 Flaming Burst crossbow.
Tactics;
Each buff themselves as much as possible. With five of six as some sort of spell caster, they and their mounts should glow like Christmas trees
All have masterwork saddles.
All weapons are coated in Wyvern poison, since they are immune to poison.
Have the Wizard create/summon/controll some undead creature or swamp beast, and wander it in to the PC's camp in the middle of the night. This will deplete spells/HPs and interupt sleep for the party spellcasters.
If being seen is unavoidable, they will fly single file, the leader in front if approching, in the rear if retreating. He and his mount will be the toughest, and most capable of taking hits, and this formation will provide the others cover bonuses to AC untill the last minute.
With long range archers in the Ranger and Rogue, and a Wizard with Enlarge Spell, they should have double the range on the PC's. Also, with winged mounts, the can run away fast, or close the distance quickly, thus minimizing the archers they oppose.
Attempting to catch the group by surprise, the spell casters will lob area of effect spells into camp, targeting not just PCs, but mounts and unattended items for quick destruction. While the Rogue sneaks into place or charges in behind the others, the two Familiars, illusioned to look like Wyverns or other creatures, will fly into camp. They will have Cat's Grace and Mirror Image. so as to look like a mass of creatures, (a dozen or more, easily), and soak up the initial hits. They will stay only one round, then flee, provoking AoO if any images remain. The leader and Ranger will enter with the familiars, charging separate PCs with lances, which they will drop after the attack for their standard arms. Between Smites and the Rhino armor, at least two PCs should need immediate healing, tying up the party Cleric from turning. The leader will then set to destroying any weapon he can, especially if this is a quick, 2-3 round hit and fade to soften them up for later.
The orc Cleric will enter combat with the leader. Each round, he will use a Quickened turn attempt at the max Heighten penalty. This will ensure that one of their number gains at least a +1 turn save. He will then use Cause Wound spells, preferably Circle of variants if you are using 3.5 rules. This will heal his people while possibly harming the PCs. He may also use Profane spells, which will cause less damage to them, since they are undead, and which the PC's will have weaker defenses against. He uses his mount to give others flanking bonuses.
The Sorcerer supports with spell power, probably getting to close to the fray for his own good, (an Achilles’ heel I'd throw in since you want this to be a dangerous fight). The Wizard drops area effect Coldballs on top of every one, since undead are immune to cold. They both hold off on these for the one round their familiars are in harms way. If the battle goes on past their spell power, due to a high number of pre fight buffs, multiple daily skirmishes, ect, they will begin a coordinated air attack. One will charge his Wyvern in to attack while casting true strike on himself while the other fires from a distance. They will alternate as needed, and stay within one rounds flight of the battle, to evacuate someone who loses a mount.
The Rogue is the wild card who could do ranged, melee, or magic from items as needed. Specific buffs, such as Pro from Elements, are pretty standard and easy to come up with.