There are several ways to get crests. I suggest you attack lvl 5 wilds. If you dont mind losing 50 archers every attack, you can send 2k archers. If you do mind, send any unit before the archers (make sure the 1 unit attacks before the archers). Btw, if you do send a unit b4 the archers, ur chance of finding crests drops dramatically. So just attack wilds over and over again and hope you get lucky.
Note: There have been rumors where you can attack the same wild over and over again many times, even if the wild has 0 units, and you can still get crests. I have not ever found this true yet, but its a possibility.