Keep in mind that I'm not familiar with D&D rules...
Have what appears to be a basic quest to clear out some goblins or other low-hp, mass-attack critters. Unbeknownst to the party (or the people hiring them), this particular goblin tribe is being ruled by a human (any class would do, although I wouldn't be opposed to the leader being a Sorcerer Lich named Xykon [http://www.giantitp.com/comics/oots0001.html]) with knowledge of the various strengths and weaknesses of the different adventurer classes, and has trained the goblins to counter them using simple tactics.
For example: When a big fight breaks out, wizards will cast all sorts of buffs on themselves (Bark Skin, Flight, Invisibility, etc). So the PCs hear a whistle blow and the goblins launch a massive attack, but appear to be split off in small groups and don't bunch up, so that area-effect spells will have limited use. When the melee fighters try to engage, the goblins scatter from them and try to avoid engagement. Archer and spear-throwers attack if they're not engaged, and evade instead of fighting if they become the focus of an attack. Once the magic-users finish casting their buffs and start to actively go on the offensive, a whistle blows again and the goblins all retreat. An hour or so later, this repeats. Continuously. Day and night. Every effort is made to prevent the magic users from resting long enough to recharge, and to prevent the fighters from avoiding exhaustion.
EDITS: Fixed a couple of typos, corrected some grammar, and expanded on the last sentence of the original post a bit.