Picts are masters of guerrilla warfare. They strike and run. If a war chief loses too many warriors he may lose his rank as a chief, so they tend to strike unexpectedly, get in the surprise round with ranged attacks, rush in if they get the initiative, strike and leave. Look into the tactics of the American Iroquois Indians and you will have appropriate tactics for the Picts. The Picts are more apt to use arrows to quietly slay a few guards, slip over the wall in the night and murder people in their sleep than they are to assault an actively defended wall (although if you get enough of them together under a shaman like Zogar Sag they will assault castles - which thankfully is a rare occurance). They will act like hunters - they will lure people outside the walls, then attack. They will lead prey into traps, and that includes human prey.
The Iroquois used to hunt deer by building a long wall with an angle in it. A group of Indians then made noise and herded the animals toward the wall. Once the animals were corralled by the wall, other Indians would pop out and shoot them down. Picts could use the same tactic. Build a palisade in the woods, then chase after a column of troops (or players), making such noise that there seemed to be more than there really are, herding them into the wall. Once stopped by the wall, other Picts, hiding in the trees, would then pelt them with arrows until the soldiers (or players) are all dead.
If you have ever read "First Blood" by David Morrell (or seen the Stallone movie), then recall how Rambo decimated a troop of deputies out in the woods. Imagine four or five of those type of guys in the woods. Playing Picts this way, I decimated a column of 32 soldiers with six Picts. Three characters managed to get out of the wilderness alive. Use their skills!
The problem with Picts is that they don't just stand there and fight. They hunt. They wait. Picts will steal over a wall, steal a woman or child, and wait until the search party comes. One by one, using guerrilla tactics, they slay the search party. Another, larger party comes. Same result. The players become afraid of the Picts not because they are such superb fighters, but because they never know when the attack will come - and it almost always comes when they are not prepared. It will come when a character has to use the bathroom. It will come while they are eating. It will come as they sleep. It will come as they are distracted by a noise elsewhere (such as the attack in The Black Stranger). It may just be a single arrow shot from the trees, or it may be a bull rush, where the character is pushed through a gantlet of waiting Picts, who then vanish when the other characters get into it.
