Engage the Enemy

Engage the Enemy is Disc 1, Track 9 on the Xenoblade Original Soundtrack. It plays during cutscenes before and after important boss fights in the game:
 * Metal Face attacking Fiora in Colony 9
 * Before the battle against Xord in the Ether Mine
 * The cutscene between the two phases of the Leone Telethia
 * Metal Face appearing on Prison Island
 * After the fight with Mumkhar on Valak Mountain
 * After the battle with Mumkhar in Sword Valley
 * The battle between Zanza and Meyneth in the Mechonis Core
 * Between the two phases of the battle against Dickson

It also plays in Xenoblade Chronicles: Future Connected, when the Telethia destabilise the Fog King's rift, allowing Melia to finish it off.

Trivia
There exists a post-battle dialogue in Xenoblade Chronicles 2 between Ursula and Rex during which they sing a tune that sound very similar to Engage the Enemy. There is also a similar dialogue with Electra.