In the Wild West booth, you will come to the Rootin’ Tootin’ Targetin’ mini game.
The UFO event does not happen till the end of the level. The UFO will start to rise from the middle of the level, with a laser beam coming from under it which will have a few target with points on them , 100, 200, 500, 1000, 5000. Hold RIGHT TRIGGER to rapid-fire and hit every target, try and hit the high value targets as soon as they come up. '
Video credit goes to OnceFallenPS3T