NINJA GAIDEN Σ

An Ambush is a challenge where Ryu will face off against a large number of enemies in succession and receive a valuable reward if the challenge can be completed. On Normal, there will be 60 enemies, each level of difficulty adds another 20 and may change which enemies are present. There are ambushes in the following chapters, with the enemies given the spawns on Normal:

Chapter 3 - MSAT Commandos - Electronically Locked Door at the opposite end of the first corridor to the door that needs the Wing Key. Drops a Lives of the Thousand Gods.
Chapter 5 - Lesser Shadow Demons - Monastery Annex, where Ryu begins Chapter 7. From the Muramasa statue, wall jump up and follow the path. Drops a Hairstyle.
Chapter 7 - Lesser Shadow Demons/Monk Fiends - Archives after 1st visit. Drops a Spirit of the Devils.
Chapter 8 - Lesser Shadow Demons/Galla - Military Gate. Drops a Spirit of the Devils.
Chapter 9 - Ghuls - Ritual Room, where you got the chalice that awoke the Bone Dragon. Drops a Lives of the Thousand Gods
Chapter 12 - Flare Shadow Demons - Peristyle Passage. Drops a Lives of the Thousand Gods
Chapter 12 - Beetle Fiends - Aqueducts 4B (the first time will have one chest and 30. Come down again for the challenge). Drops a Talisman of Rebirth,
Chapter 14 - Samurai/Samurai Mage - Hayabusa Village, on the bridge where Ryu fought the boss of Chapter 2. Drops a Hairstyle.
Chapter 15 - Galla - Zarkhan Stadium, use the switch then turn around and head back to the big open area. Drops a Lives of the Thousand Gods.
Chapter 15 - Lesser Shadow Demons/Flare Shadow Demons - Stairway to The Imperial Palace. Drops a Lives of the Thousand Gods
Chapter 16 - Wasp Fiends - Underground Passage, where you have to run around the walls to reach the top. Drops a Jewel of the Demon Seal
Chapter 16 - (3) Military Ghuls - Drawbridge Square. Drops a Talisman of Rebirth.
Chapter 16 - Flare Shadow Demons/Auriochs - Great Bridge Square, use the portal in one of the buildings off to the side. Drops a Talisman of Rebirth.