I saw a guy at my local store using a flying Nurgle Daemon Prince with ML3 the other day, and boy did it clean up. With Jink and Shrouded (from MoN) it had a perpetual 2+ cover save, it has (at least - glide vs swoop) 12" movement, and could chuck out horrible amounts of psychic hurt even while jinking because there seem to be no restrictions on manifesting powers after electing to jink (unlike with shooting attacks which can only snap-shoot). It could choose to swoop and force snap shots against it, or glide and assault stuff.
1. Is this legal? (it certainly seems to be...)
2. What can we do against it?
On a 4'x4' board it's not easily avoided if gliding and our lack of Skyfire means its very hard to hit while swooping. Our poison mitigates its T6, which is a start, but there's still that pesky 2+ cover save.