The loss leaves the Elephants in third place in Group A, trailing behind Equatorial Guinea and Nigeria, with only three points.
Their hopes of advancing as one of the best four third-placed teams in their respective groups appear very slim.
Emilio Nsue, the top scorer of the Nations Cup, found the net twice in the 42nd and 76th minutes, increasing his goal tally in the competition.
Pablo Ganet added a third goal in the 73rd minute, and Jannick Buyla completed the rout with a goal in the final two minutes, sealing a nightmarish outcome for the Elephants.
