We have only men and beasts as examples in the real world, but in every case, the organized have defeated the unorganized, after adjusting for other considerations (absolute force strength, surprise, the weather, etc.). Group coordination, discipline, and leadership appear to be demonstrably superior to lack thereof. Whatever mundane edge you give the unorganized side -- high ground, martial training, surprise, weapons -- would benefit the organized side as much or more. Thus, I'm with DrTemp on this one: you have to postulate extraordinary abilities that specifically only work when
not fighting under a leader and in formation.
In
GURPS terms, an entire race with Daredevil and Higher Purpose (Eschew group-think to achieve individual glory) would probably do it; you wouldn't
have to go as far as exotic powers. Getting a net +2 to die rolls and being able to reroll critical failures whenever you charge the enemy like a madman and ignore all discipline and tactical sense might be enough to offset the lack of cohesion -- especially if those doing the charging are also strong and able to take a few hits (so toss in +1 ST and +1 HT, say).
Mad Berserks0 points
Attributes: ST+1 [10]; IQ-1 [-20]; HT+1 [10].
Advantages: Daredevil [15]; Higher Purpose (Eschew group-think to achieve individual glory) [5].
Disadvantages: Berserk (12; Battle Rage, +50%) [-15]; Selfish (12) [-5].