I think there maybe a problem with the group thing were others cannot attack a monster. A momentous had spawned out to my claim me and my wife got geared up to attack him and when we were ready we started fighting him " we had forgot to /group invite each other. " just after attacking him i started getting the message that it was my wife's mob so i did /invite command to have her join a party. and she did the /join command. after she joined my party she began getting the same message that it was TemperedAngels Monster and nether of us could attack it. After seeing this we both did /group leave and tried having her do the /invite command. It did not help and we had to wait the 15 minutes until he became free. Don't know if this is a bug or not but figured i would let someone know.. Thanks.
You need to pm the developers with bug reports. This is an interesting side effect of the new system though. Were you ever able to hit it or did you try again later?
We both waited out the timer but he ended up taking damage due to water and we lost effective difficulties down to 4