Depends on the build...
There's a couple of life creature rushes in it that are very fast. Those can be hard, if you draw your spell cards late. This is a reason I use a proportionate deck most of the time... 10 attackers and 9 spells.
The ray of light + fractal + hope is also awful, but then if you have some lightning/otyughs, it means nothing.
I do not know all the names, but some buffed lava golem decks are pretty tough... especially if they use those cremations and novas and attack fast. Any deck with antimatter, maxwells, and/or a couple of fire storms is tough too. The whole combination kills most of your creatures...
And finally: Ice shield/fog shield/dusk shield/phase shield + SoG shards. These decks can be countered with the pulverizer, or you can just be really fast.
I know it's a lot, there's just not ''1 hard deck for me.''
