From extensive grinding against Eira, the most consistent decks I've found from the ones posted so far are the first ninja deck (the one with the demon slayer and necromancer) and the earth deck with the jiangshi.
I'm not sure if the translation was different for some reason, but if you mean the A rank master with 26000 HP/2600 attack (mine translated it to Eilira), I've beaten her on hard mode with my stats capped at 6000/1000 with something like this (obviously, this deck is designed pretty much specifically for her - in general the deck makes no sense, but it's good at countering what she does). Pretty much my logic is that wind, spells, and higher level monsters suck against her (it wasn't translating for me, but it seemed like one of her permanent spells turns all spells into wind type and a lot of bad stuff happens when you use wind type, and she usually kills any monsters you play the turn you play them so you never get the chance to play the higher level stuff), but traps are fine, and higher level monsters are awkward because she usually kills whatever you play each turn, and abusing the trap that draws cards whenever wind effects happen (you don't need any wind effects because she'll trigger it for you). Mirror shield is also really strong against her because she'll be making a really high damage attack every turn, so I more or less designed a deck around those constraints.
I guess if you were grinding you could set it to very easy, in which case you can probably win with almost anything because you get insane stat buffs. Her AI also has a tendency to bug out when her field is full with spells/traps (she'll stop playing monsters I think because she wants to play a spell that summons the monster instead, but she can't play the spell because her field is full.. so she kind of just stops doing anything).
You can still randomly lose sometimes when she gets a good draw - I've had a start where I summoned 2 monsters, played 3 traps on turn 1, and then on her turn 1 she killed all 3 traps before attacking, both my monsters and brought me down to 0 HP all on the first turn before.. and I can't imagine that anything would be able to deal with that kind of nonsense.. but if you survive the initial onslaught you'll usually be able to win the battle of attrition.
