Discover and fight Hidden Kombatant 4 in Arcade Ladder
This can only be done on the Temple Map. Make sure to set the amount of rounds to two (standard settings). You can make changes in the options section of the main menu. Play Ladder Mode as long as you get to the shown map. This might take some while and is pure luck. Try to combine it with other Trophies / Achievements or keep restarting Ladder Mode if you are not willing to fight your way through over and over again. When playing on the Temple Map you have to fulfill two relatively simple tasks. Win both rounds (in other words: don't lose) and don't use your Block ability. Don't even think about pressing the button. Moreover you have to hurry up a bit. In the second round you will see 'Noob Saibot' (the black colored guy) standing in the background. He is the hidden Kombatant. Win the match while he is standing there.