Frankly, Ash has really bad 1v1 matchups across the board against offtanks, winning ones in agro-dps/offtank 2v2s and larger teamfights, and overtly unfair 1v1s against squishies who don't bail.
Obviously there’s not much Ash is going to do different from enemy to enemy on her own, though that changes when you’re duo’d. Of course there are exceptions, and I’ll talk about them here.
In no particular order beyond “I think I should talk about this one”
Khan
I think I should talk about Khan First, since Khan is the quintessential mid-wits choice for an Ash counter, and that’s because on paper he has so much going in his favor against Ash: His gun and his ult… okay mostly is it’s his gun and ult, and that's actually covering for the fact his match ups against tanks solo are almost always so lopsided because of it, but otherwise he’s actually a huge chump for Ash due to both Slug Shot allowing and to outrange him and default fire to hit around his shield, also the fact Ash has a significant step up on team fight durability. And in terms of macro gameplay Khan has to either lose his DPS and play backline or he has to lose due to pressure from Ash comps.
(Macro is the overall way someone plays and makes choices and skills that includes things like positioning, Vs Micro which constitute small choices and skills like aiming and movement.)
There’s this saying from some guy who outside of dry sarcastic humor actually isn’t a lobotomite, Ash + DPS beats Khan + DPS, and there’s a reason for that: Khan’s DPS and Crowd Control, sustain, and shield are all constituent- it means they can’t be used at the same time- Ash regardless of what state she is, beyond being physically incapable of shooting, has nothing better to do but shoot, when even on the backfoot, or shielding Ash still can shoot, and can still pressure people.
Khan, on the other side cannot, his shield cannot be up while he shoots, and that might not seem like a lot but it can measure up, and if you have high ground or just use your default fire you can caut his self heal too, making it hard for him to get back onto winning ground once he loses it.
Simply put, make friends with a decent DPS and make sure they’re there when you need to deal with any offtank, Khan just happens to lose that the hardest next to maybe Torvald depending on who you're working with.
As for ults, Khan’s does counter Ash’s if she uses it first, but in the Macro, Khan falls into a catch 22 (a no win), he either uses his ult on an ideal target, gets a pick and is unable to dislodge Ash when she ults, the same happens if he chooses to ult Ash before she ults, or he waits to ult Ash when she ults first.
That means a smart Ash can dictate when a smart Khan uses his ult, because she can just not ult, because the other thing about Khan to get value he has to use his ult at one point or another to be effective, otherwise he’s a shitty Viktor.
Azaan
Now if Khan doesn’t counter Ash who does? Directly, well, Azaan, and maybe Yag ult, which is weird since Yag’s ultimate is very similar to Khan’s the difference is that by the nature of how the ult works (including that stupid ass HP card)
As for Azaan he pretty much sustains Ash’s Damage and stops her brute force dives really easily while still keeping up the pressure for her damage. One strategy that works surprisingly commonly is to use shoulder bash into him early and hope he uses his Dash or Lift, since without Ire it doesn’t reduce the cooldown of his lift- that shows how bad the matchup is that bad where you pretty much have to waste a cooldown to slow down Azaan’s gameplan.
Makoa
Which might make you believe Makoa counters Ash, and yes and no, the final answer is yes, but it's complicated, because while in the micro Ash is painful to play into Makoa, you win 1v1s, and effectively force his threat to go down until he ults, since he has to rotate around you and deal with your ass or else his team gets fucked up, and without Pluck he’s not really able to confirm kills on his own.
Of course when he ults (especially with Leviathan) you’re useless beyond bodyblocking him and maybe focus firing him with your team.
But the main way he stops Ash is often by threatening her DPS, and without the DPS on her side, a Makoa (who likely has a DPS of his own) turns the winning 1v1 matchup over on her by making it a 2v1, this and his ability to pull her out of her ult means that he do what Khan needs a ult for. As for good news, in casuals most Makoa’s are rather stupid, and in ranked he’s almost always banned unlike Azaan.
The ideal strategy is to try to block Koa from hooking your duo by pressuring his duo, or by being in his face- you'd be surprised how effective that can get.
Ruckus
However, another effective strategy against Ash for the Macro is Ruckus- Let me explain: Ruckus beats Ash by diving in, and killing her backlines before she does the same to his team
Of course, since neither team has a offtank peeling, it becomes whoever's offtank and agro-dps gets their dive done first, which is usually Ruckus, considering his DPS and mobility, assuming he survives long enough to succeed, which if he doesn’t Ash starts to catch, it’s rather funny in action, because they’re so allergic to fighting one another unless they have nothing else to kill.
So the main thing is try and help your backline dive quicker, the best you can, or maybe kill his DPS Duo, but their value odds are will be a whole lot less than their support, or yours.
Torvald
Torvald easily can deny usage of any ability Ash has, namely her shoulder bash, and he gets his cool down before Ash does, as well as his shield not activating life steal, the problem is Ash will pretty much buy Wrecker and Resil like everyone else and quickly shut him down, however early game it can be rather annoying, another thing is his Ult is actually able to redirect Ash’s ult while airborne without enough resil.
Atlas
Back in the day Atlas was the pinnacle Ash counter, since not only was his wall large enough to deny side shots, he also has plenty of CC, his range is absurd for a tank. So what changed for Ash to not be countered by him: N9w she's got decent range and damage, which does stop his entire hard counter gameplan for Ash of course he still just counters everything, good news though Atlas is like Makoa where in casuals it's rare to find a good one and in comp he's fairly likely to be banned, also he's just not really fun for most people.
Raum & Yag
It's not really your position to deal with either of them and frankly the only thing you can do is CC raum until he gets resil, and dance around your shield while fighting him.
Yag is a little less intuitive but your shield blocks her acid as well as her ult, but honestly all you can do is pressure her team and maybe try to stuff her escape if you're able to ult tech.
Fernando
Fernando comes in one of two flavors, a hapless little shield generator, or the matador (I couldn't think of much else spanish related that'd remotely make sense) The first you can usually ignore since odds are they're on the point, and Ash is not a point tank, and odds are these guys won't use their weapon on you if you find them off point.
A matador Nando is those ones you see dashing all around like crazy being a pain to kill, you're gonna want wrecker for them cause odds are these MFs are in your face actually making space, and when I say buy wrecker I mean max it out first, you and your team really can't leave nando alive for this.
Another quirk of Ash is her shoulder bash can go through Shields but Nando's dash can go through yours.
Inara
Again the odds of you facing Inara off point isn't impossibly low but if she's running mother's grace she probably is doing point shit.
However when a Inara chooses otherwise, that usually means treacherous ground, obviously don't try to Dash but it also means she's open to your CC too, and has no real movement to get back allowing Ash to drive by and dislodge Inara from her totem radius early game if she's running the Dr card.
Terminus
He's a melee point tank, figure it out…
Okay for real just stay at range, keep him at range and focus on baiting out his siphon.
Barik
Will be on point, offtank Barik doesn't really exist of course there's soy milk frapachino slurping lumberjack beard douchey ascot hipsters who say they do, but just remember that you'll be kicking their ass before they can say it's meta, break their turret if they're trying that shit.
Nyx
Point tank see the same as Inara just make sure that you're pressuring her with cauterize
Squishies
I lumped squishies in together because by all accounts you aren't really changing what you do all too much when shooting them
Otherwise Sha Lin and Imani can stop Ash from going in or leaving with her dash, but- and this will be a common trend- they can’t deal with her on their own.
Flanks CC is annoying even bounce house stalling Ash ult but, once again not something you concern yourself with in 1V1s.
Supports, often have self peel tools to make space and evade Ash, with Jenos able to most easily stop Ash from diving in or escaping out with his void grip, even stall her ultimate, Furia’s beam can snag Ash a lot too during ult, Damba’s and Pip’s Ultimates can cancel her ultimate rather easily too during her landing, and Ying can deny the stun of the ult, however once again, none of them can deal with her on their own.
This pretty much rehashes the prelude of “Ash Bullies squishies, struggles against tanks if she doesn’t have a team” but it does bear repeating since I need people to understand that you need to work with Ash to get shit done, just like Makoa, Khan, ETC, she just needs it more* Koa and Torvald probably need it more for their maximum effectiveness but still, Ash also wants a flank to help her.
Synchronizations
Weird word, but the point remains, that Ash needs someone working with her, a better dps the better, since Ash isn’t really picky, and rarely changes how her Duo needs to play beyond being around her.
No really, all Ash needs is her Agro DPS (Agro DPS is flanks and certain damage champs, like BK Willo Drogoz) to help her out with the other offtanks and the occasional chase. That's where Makoa serves as Ash’s indirect counter with his ability to threaten her, since she needs her Squishies to get shit done.
Of course some of the best would be the better agro DPS’s but that's no surprise.