If you don't like being attacked you can do like I do.
have a handful of 5 star monsters that are decent level to deter people, then set my leader to a harpy (three stars). The people who do attack me have no idea what they're getting into so they get obliterated. the higher level players with teams of seven stars don't bother hitting me.
Obviously I only do this when my leader doesn't matter and it doesn't inconvenience my allies.
The end result is that my defense is successful and I get 100 brave rather than losing 10,000 luna, not that it matters. It's just funny writing "CAAAAWWWWW!!!" on the walls of people who attack the harpy.