Definitely the hardest time I had was P1 hero pressure and carefully playing around upgrades, with a special shout-out to Hardened Mox + Boot Camp + Drakk that @Penatronic rammed me with repeatedly. My losses were to Pena on Present Peace Blood, where that Mox got tons of value and I would have loved to have Plague Spitter or Orpal, and PetE’s Anarchy/Str/Growth, where I couldn’t stop MoLaC in time. But as it is a fairly hero-centric deck, definitely the scariest thing to do was be P2 against a strong hero opening, where I had to hide my heroes the first few turns to avoid giving up levels.
This is a known “very good” deck, nothing of my own creation, just something I wanted to try out for myself as I hadn’t actually played with it much. I don’t plan on playing this in tournaments regularly.
As for Nether Drain, it makes the hero lose levels, which often means losing stats and abilities. But they just become the lower statted, lowered ability version of themselves.
For example, a 4/6 rook with 5 damage Rook at lvl 8 with no crumbling rune gets Nether Drained, he is now a 3/5 with 5 damage that doesn’t have two lives; he dies from the Nether Drain, and as the nether drain caster I now have 4 levels to give to my own heroes (2 for the drain, and 2 for the kill).
If you have more specific interaction questions, please just let me know!