Originally Posted by Roentgen

Are they the ones that need to be killed within 15 minutes? (or so I read...)

Yes. I think they've been in an event only once so far though, and quite a while ago too. I think it was around the Star Lily event, but I might be 1-2 events off. At the time, very few people had maxed even their helper squad, so they were pretty difficult to kill. Nowadays, they should be easier to handle, provided most attackers go all-out against them with full (3 raid-point) attacks.