I think you're approaching this encounter from the wrong angle, maggot. I would not have the orcs attack the party. I would have the party attack the orcs. I agree with an earlier poster--orcs are not so stupid that they can't look at a group of obviously superior foes in shining armor and glowing cloaks with levitating sorcerers on phantom steeds and fail to get the point. However, you can still build this encounter in a way that is a satisfying "F*ck yeah!" to the players for all the hardships these orc raiders put on them in their early years.
If I were designing this encounter, I would have the traveling PCs come across a burned-out village, still smoldering, that had been ravaged by an orc raid. I would make it grim, with burnt corpses of little girls with pigtails. I would make the size, strength, and disposition of the orc raiders patently obvious, as well as the direction in which they had headed with captured loot and slaves. I would prepare an orc lair with, say, 200 orcs in a tribe, including a 6th-level chieftain, 4th-level shaman, etc. on down the line to the orc females and whelps. Then I would simply sit back and watch the fireworks as the PCs put some vengeful, well-deserved, and oh-so-sweet arse-whupping on the unsuspecting orcs. I would make sure that a few members of the tribe survived the PCs' onslaught (probably because they weren't present at the time), and I would allow those orcs to circulate rumors throughout the region about the terrifying, godlike orc-slaying devils. I would then, after an appropriate amount of in-game time, allow those rumors to reach the ears of the PCs. Nothing will sound so sweet to them as that.
That's how I'd do it. Think big, man.