Sorry for necro-ing, but I really want to share some of my opinions on ghostmares:
1. Nightmare. The AI tends to play it on your first high-cost creature before they even got a ghost out. This makes ghostmares a mere mono time rush/time-darkness duo with steals.
2. SNbows. SNbows are very common arena grinders for every league. Is it that hard to assemble 6/7
and make ghost backfire against your opponent?
3. Obviousness. You know it's a ghostmare just by the oracle card and mark most of the time. The repetitive strategy of ghostmare can be countered by some careful planning (e.g. leave some cards in hand for discarding, save up
to paly a ghost, lure early nightmare on your own creature etc)
After all, ghostmares are not that hard to beat... I personally defeat them in gold league 70% of the time
P.S. Sanctuaries, a common defence card in rainbows, is a hard counter to ghostmares, as mentioned before.