Just keep using skills repeatedly to earn experience towards this. The fastest ones to earn this on are X-Ray Vision (1MP, useable in combat and does not use up a sub-turn) and Magic Light (1MP, useable at all times), both of which can be obtained very early in the game from a vendor.
For a quick grind, you can use all your MP on Magic Light while standing next to a healer. Just pay to recover your mana and repeat until you get the trophy.