Special skills are the unique abilities mapped to R1 + symbols, set through the Pre-Battle screen based on what you have access to. The power is built up through hitting things that are still active/alive, so keeping a hit combo going on corpses will not generate more power. As you build up the power, you'll hear a small chime from the ability or abilities reaching their full potential and becoming available. After that, you have to hold the R1 button and press the appropriate button to trigger the ability. Some abilities can be used in the air, though most will require you to be on the ground. Find whatever works best for you and make sure you use your skills often in a single battle to unlock this trophy.