Speak to Stanford in the Eversun City Inn (X:168 Y:276). Stanford will ask you to get a team of two other mercenaries of any level.
When you've gotten them together, speak to Stanford again.
FIGHT: Go to X:560 Y:355 in Placid Plain. After the cutscene, you will first fight three Shadow Assassins (level 24 non-elemental Mercenaries, 1966 HP) and Speedy Shadow (14656 HP). Upon defeating them, Shadow (level 30 non-elemental Martial Artist, 15778 HP) and Night Shadow (level 30 non-elemental Mercenary, 14344 HP) will appear.
When you've defeated them, you will receive an Official Seal. Take that and your entire team back to Stanford in the Inn to collect your rewards.