I believe it is pretty straightforward:
Make a leadership check. When successful (Rolling 111+) applying modifiers based on the target race, morale and other factors , allow a Fear RR based on the Leader's lvl vs target lvl. Depending on the amount of failure he'd be at -10 to OB and DB for 1-10 rounds(failing with 1-25), flee for several rounds (1-6) (failing with 26-50) or be paralyzed with fear (51+ failure) for 1-6 rounds)
Of course never allow Constructs and undead to be affected, the same goes for even low level giant creatures, or non-humanoids without sentience. (To a basilisk a snarling human may just be the dinner bell, and to a young giant an interesting toy.) Then there's the battlefield factor: surrounded by a few dozen of his trusted toadies, the half orc chief will not be impressed, or at least: very hard to impress.
Game on!
