It is infested with Ambre 90% of decks have her

Ouch.

Use a Sakrohm half always!!

graks, Uranus will brace yourself vs Ambre hits
use can use trin as well if you want (if you want a higher chance of Sakrohm bonus to draw a pill but against Nightmare...).

for Nightmare vs Nightmare though?

Glorg is a good choice with Ambre support and can smash through anything (and I mean anything...)

Azgroth is an okay counter vs Ambre powered things.

Nistarok is like a Nightmare graks vs them.

And fill the rest with OP Nightmare cards for your deck.

special mention to Oshitsune vs Oshitsune though

even with Ambre support another Oshitsune can match his power so make sure you remember that...

good luck

I play Ambre
Glorg Azgroth Ghumbo Oshitsune
Caelus Dwan (for DR) Tomas

Even with all the Glorg SOA floating around, seeing Ambre , Caelus and Oshitsune across the playing field gives the opponent pause for thought.
By the way, I often pill Dwan which surprises the opponent, who usually assumes she's just there to soak up damage. One could do the same with Nistarok.

My tips

1 - elo survivor - less damage dealt each round os more chance for you to win a round (lets face it T2 is a 2 round ko area)
2 - Mawpin, Eadh, Sargh, Azgroth give you the best chance of winning
3 - i wouldnt use a Sakrohm half in elo but that is just me, id prefer mono

and they were my 3 tips to success, trust me elo is miles quicker than T2 i switched after 50 wins and it was over in a few hours, i love my dudley