This paper is a retrospective case study of a game-based learning (GBL) researcher who cooperated with a professional gamer and a team of game developers to design and develop a coaching system for First-Person Shooter (FPS) players. The GBL researcher intended to verify the ecological validity of a model of cooperation; the developers wanted to assist FPS players in overcoming a bottleneck that hindered players from becoming professional gamers; while the professional gamer desired to venture into professional game coaching. The synergy generated by these individuals resulted the creation of FPS Trainer. The key challenge encountered in producing the system was to make in-game coaching and learning outcomes explicit while preserving FPS Trainer as a fun game. This paper illustrates how the challenge was overcome and discusses lessons learnt from the case study. The outcomes of the case study would benefit academics or game developers who plan to initiate cross-disciplinary cooperation for making coaching or training games.