How I see this is not a matter of being able have a few dozen troops, or indeed even hundreds, in hiding somewhere waiting to do a bit of mischief, either leading up to, or on the peripherals of a battle, but rather a unit that while positioned out of the enemy’s line of sight, and likewise with orders not to become involved in the battle until a certain time/something happens, can if successful have a major impact on the whole thing.
ie A strong body of archers suddenly being revealed at a set point of the battle, perhaps on their sides middle rank when the attackers reach their first, as something like that could have a marked effect out of proportion to the numbers involved, and similarly was the kind of thing that happened.
I also think the unit involved could be thought of simply as your reserve, as likewise need have high stats training wise ect, or even/and commanded by someone with high stats themselves.
