OkMany Vanilla projectiles can be used against Selenian such as rockets or a limited selection of Melee, Summoning, and Magic weapons. For modding purposes, I'm not quite sure, that'd be something to put research in.
If you have posted this thread because you have trouble with Selenians, you could try using rockets(NOT Electrosphere Launcher) with a powerful launcher to stop reflecting, or by being a bit more careful when around one. They are quite tricky indeed, so stay on your toes.