Ive lost a couple times already, so it is not unbeatable(nothing really is). Here is what has beaten me. Keep in mind I made mistakes in these matches so you WILL have to outplay them to win.
EN with firete or janus.
Emma, it crushes me, like really bad lol.
WK with vol or other character hate cards, the second part is crucial.
sp manipulation decks, this one surprised me a little, but if i don't have the sp to play anything then I guess I can lose lol. You better have good timing though.
Some people have also come close and probably would have won if they didn't make some mistakes(mainly dealing with dilate)
The one you saw, priests deck.
Mermaids with those supper buffers, I could have lost easy if I didn't win some key agility rolls.
Rushes with shield of the weak / absolute wall type cards to counter ruuca, even though I can spam it indefinitely while they can only play it 3 turns, if they had a better field before they started using it they could have won.
Hope that helps people, it is still op though, no doubt about it. Like I said you will probably have to outplay them to win. The only decks that I can see that really counters it is EN, character hate, and emma
. Everything else will be an uphill battle.