The core is only exposed during the Cannon attack. Watch out for it and stun the Ruin Defender while it charges that attack.
The Ruin Defender is not particularly threatening, but the Shield can make it annoying to kill. It's best to focus other enemies during it's Shield ability and let it come to you after it's done.
This attack is only used when Shield is active, and only if you enter **melee **range. There's about a 10 second cooldown before this ability can be used again.