How to fulfill 'Gain a Skill Level in any Art Skill' in Art Society?

Hi, I know my sim can practice any musical instruments, or photography or paintings. But unfortunately, my sim is all maxed with Paintings, Piano, Violin, Guitar, Photography and pipe organ. So there's no art skill he can gain..any more. Then how can he fulfill this mission? Should I just leave it as undone? I want to get a tip for this. Thanks. 🙂