Discover and fight Hidden Kombatant 2 in Arcade Ladder
This can only be done while playing the Forest map in Ladder Mode. Usually, right after the beginning of the match you can see the Character 'Smoke' behind a tree on the far right. Press Select + Down (PS3) or Back + Down (X360) and the fight will end immediately. Instead of your current opponent, you will fight against Smoke and the Trophy / Achievement will pop as soon as the first round starts. It is possible that Smoke pops out from other trees but the shown one will work in most cases. The following trick can help you with all the Hidden Kombatants:

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If you fail, you do not have to start all over again! Finish the match and take your enemy out with a normal attack (no fatality!). Now you will receive your koins. Pause the game immediately and choose 'Player Select'. It won't be count as a deafeat then. Of course, you can now choose an other fighter as well or simply continue with your last one.