I see. Did they buff Eiraira-sensei's deck in one of the patches? I feel like she's suddenly way harder and the deck more consistent; I thought I used to be able to take over 50% of the games I played against her but now I'm losing like 80% with even solid decks. Her ability to destroy all your magic and trap cards with Gunner Harpy and Falon (and the randomly very relevant ability to turn your stuff into Wind-cards) effectively neuter a lot of otherwise strong archetypes; Reservoir, Hiding, Ocean Voyage, etc. have trouble landing the strong creatures you need to really take it home. Even stopping her spells with Magical Frozen and Tsunami doesn't really do that much due to how heavily creature-based her deck is - best-performing deck I could find against her thus far was ironically a Wind-deck of my own (mostly thanks to it not really needing spells that stick in play to get a bunch of stuff out) but I don't think I've yet found the right build of that deck. And it's basically a mirror making it a bit tough since her cards are generally better than mine.
She's always been like that (at least on hard mode). I think you probably just got lucky before (or maybe you weren't on hard mode - I don't know how different she is on lower difficulties).
The best strategy for beating her in my opinion is to try to only play spells on the first turn and never play continiuous spells or traps (except as an act of desperation), and try to only have 1 monster in play at the end of each turn. The goal is to get her to fill her entire board with spells/traps that will never trigger, and once she has 3 or more spells/traps she can't play any more spells, at which point the only thing she can do is just play 1 4 star harpy each turn, which you can kill each turn with ninjas and eventually run her out of monsters she can use (she only has 6 4 star harpies in her deck).
The main things to be wary of - every time a spell or trap card is destroyed, she can summon her 6 star harpy by discarding a card. This is the main reason why you don't want to have traps or any continuous spell in play. It's also why destroying her spells isn't very effective because it also triggers when her spells are destroyed too. Additionally, this also triggers when her trap that draws cards whenever wind effects are triggered 3 times - this is largely why I put so much emphasis on trying to prevent her from triggering her effects, because you absolutely do not want that trap to draw cards 3 times.
Second, if you have multiple monsters in play, and she kills one of them while she has a gunner harpy in play, she can trigger her effect to do damage to the other monster (it won't trigger if you have no monsters left). This is why you don't want to keep multiple monsters around, because if that effect triggers it triggers the wind trap that draws her cards - and as mentioned above, if that triggers 3 times, she gets to special summon all of her 6 star harpies and things can go south very fast.
Third, using spells.. this can *sometimes* be worth the risk, but there are a few things to bear in mind. First, she has the continuous spell that triggers whenever you cast a spell tand reduces the next damage a harpy takes by half - the damage reduction in itself is no big deal so if that's the only spell she has in play you don't necessarily need to stop playing spells, but once again, if that triggers, it means that it triggers her traps that draws cards (as well as the healer harpies too). Secondly, she also has a trap that removes 2 cards in your hand - I'm not really sure exactly what the trigger for that trap is (the translator didn't work on that card for me) because it seems to happen on some other effects too, but I know it definitely triggers whenever you activate a spell or trap, and you want to avoid triggering that when possible (not only because of all the bad stuff that happens because of the effect itself but also because your goal is to fill her entire board with spells/traps). She only has 1 copy of that trap in her deck so sometimes if you're desperate it can be worth gambling that she doesn't have that trap, but you want to try to avoid it when possible.
Fourth, she has some cards that change your cards into wind types. Fortunately, ninjas can counter this easily, as when you return a ninja to your hand it becomes dark type again - all of the bad stuff associated with wind types only happens when an effect is triggered, so if you play the wind ninja first (no effect is triggered just by playing it) then use the effect of a dark ninja to return the wind ninja to your hand then nothing bad happens and you turn the wind ninja back into a dark ninja and you can play normally after that.
Anyway, the deck that I use against her, winning something around 80-90% of the time on hard mode (with the caveat that I was very overleveled and there are a ton of ways to misplay it and my win rate was considerably lower before I figured out all of the nuances of the matchup - I've also beaten her with the 6000/1000 stat cap, but that I can probably only do it with the stat cap something like 30-50% of the time) - play your card draw/cycling on turn 1 obviously, and then hopefully never play any of your spells/traps after that (but if she floods the board on turn 1 and you can't deal with it then you can use whichever of explosion or mirror shield works best in the situation - if you have to then try to play all of your mirror shields in the same turn and activate all of them even if you don't need more than 1 of them - it's better to have 0 traps than 1 trap so even using mirror shield to do 0 damage can sometimes be correct.. ideally you never play the mirror shields at all, but sometimes you have no choice). Depending on the situation you can sometimes use defense mode on your ninjas - more or less what it comes down to is defense mode is better when she has no minions, attack mode is better when she does have minions, but there are way too many exceptions to give a simple answer for when you should or shouldn't use defense mode: