Focus is a level 3 Black Magic spell that lowers all enemies' Evasion by 10 points. In the GBA/PSP/iOS releases it costs 10 MP to cast. In the Pixel Remaster, it lowers Evasion by 20, as it was originally intended in the NES version.
Behind the scenes
The spell is bugged in the NES release, as it does the opposite of its intended effect by raising the enemies' Evasion by 20, making them harder to hit.