Place it at the start of the showdown scene, then follow it with the Metal Gear Alert Sound when the opponent appears. After the action ends, add a short laugh to close the scene, keeping this clip as the main cue for the confrontation.
About this sound
This clip plays about ten seconds of Megalovania, the fast, driving opening linked to Sans in Undertale. Its sharp rhythm creates a tense, dramatic feeling, like a difficult battle is about to begin. The track comes from Undertale, released in 2015, where it accompanies one of the game's most famous fights with Sans.