Fighting him is an absolute pain and he’s so hard to deal with it’s frustrating.
I play mostly Kan-Ra, Glacius and Arbiter with Gargos and RAAM as pockets.
I accepted the fact that Kan-Ra cannot win vs Rash, never tried Gargos vs him and I am not experienced enough with RAAM to fight him.
My troubles are when I pick Glacius or Arbiter since they are my only true options.
There are a fair number of “what do I do against Rash” threads floating around already - I’d recommend doing a quick search of the forums and seeing what you can find.
And also, you’ll want to state your character when asking for help, as who you play will often inform the exact counters you need to employ for things. Your icon is Gargos, but there are a fair number of people here who don’t play the character in their icon so explicitly stating is always helpful.
I edited my post to include the characters I play.
I mostly struggle with his approach options.
There is seemingly no way to stop him from getting close to me with his Tongue or intercept his Wrecking Ball.
Also I hate the fact that when I am knocked down, he can either sweep me with his c.HK or get me with a overhead with his b.HP.
And his air MK is pretty annoying too.
Rash is a pretty unorthodox characters and can be difficult to fight against sometimes. Against wrecking ball, using a characters AA option (if they have one) after you block the ball is the best option.
Lets say Kan-Ra, Sand explosion is a great get off me tool against wrecking ball and a misplaced tongue
Arbiter has his Grenade self explosion and shield pop to negate any air pressure. You can also ue your LP+LK target combo to stop any tongue pressure. You have to use it pretty quickly however. If i recall, I think tongue may be slightly negative.
Raam has it easy since his L. kryll shield can stop any wrecking ball.
When your knocked down however, there’s not much you can do apart from taking the mixup or using your wakeup option. Block low when he’s not in range for the overhead, try to react to overhead if he’s in range.
Keep in mind I don’t play rash, so I think someone who understands him better could give you more tips.
I play some Rash. Like Joker said, blocked Wrecking Ball is a free AA, or it should be. I won’t play a char without a good cr.HP
More on Wicked Tongue. Wicked Tongue inflicts a specific amount of hit or block stun (I think 25 frames hit stun, 11 frames block stun), no matter how far away he is, but his recovery increases with how long he travels to you. If a Rash tongues you from far away, you may be long recovered before he gets to you. cr.MP is the goto way to interrupt this with Hisako; you might want to lab out what your char’s best way is.
In general, Rash’s pressure is pretty interruptable. All of his specials are negative. Ram is -2 / -4 / -8 on block, and Boot is -2 / -5 / -8 on block. He doesn’t have great frame advantage on his normals either (except cr.LK and st.HP). Altogether, you can take your turn back a lot against Rash.
His overhead is pretty good, ngl. You have to deal with that mix, but note that Rash is fairly weak when he’s down too, especially meterless. If they have meter, Shadow Boot is a real reversal to watch out for, so you might want to bait that out with something.
One of the challenges of learning to fight Rash is that some of the counters are a little tricky to execute. As has been said, you CAN anti-air his wrecking ball but for whatever reason this isn’t always an easy thing to do. I main Glacius and I tried and failed to anti-air his wrecking ball a lot when encountering him online so I assumed it just didn’t work. After people convinced me that it does, I had to go back and work on the timing. It’s odd because most anti-airs want you to beat/anticipate your opponent’s attack early but wrecking ball needs you to wait and then anti-air rash after the block. It’s finnicky in a few other ways but don’t give up on it.
Good Rash players move quickly between his myriad goofy, fast, and confusing options and I think he is probably the second most frustrating character to fight against after Shago and it’s very close. I don’t know if Tusk wins the matchup overall, but his huge sweeping sword anti-air c.HP applied judiciously really shuts down the wrecking ball shenanigans (incidentally this works well against Shago too). Tusk is a little weird to play but it might help you get a better handle on Rash’a other nonsense if you had a good counter to his constant swinging around.
This is what tends to give most players trouble against Rash. The most important thing to remember is that when Rash tongues in, it is generally not his turn. Any normal that’s 8 frames or faster (cr.MP is the button HIsako uses) that covers the space he’s in will beat any of his options. Important to note that if his tongue trajectory makes him cross you up, you’ll generally just have to hold the MK that follows because your ant-air will come out wrong side. For dealing with wrecking ball, you want to block the ball itself and then do your cr.HP or other anti-air. Wrecking ball is armored and generally difficult to stuff, so you’re usually better off blocking it and getting your punish after.
To your point, there really isn’t a way to stop Rash from getting on top of you if that’s he wants to do. Your power in the MU is to be able to punish those approaches after he bullies his way into your face. A lot of the standard Rash pressure (tongue approaches, wrecking balls for days) is punishable so long as you know where the gaps are.
As Code says, you just have to hold that. All of the characters you play have similar 50/50 mixups that aren’t really reactable and that the opponent just has to deal with - KI is a game of strong offense across the board.
As you say, Rash pretty much dunks on Kan-Ra as I understand it. @nSxTyme2Dul may still be able to give you some pointers in the MU though. @SonicDolphin117 may be able to give you advice with Arbiter, and @LetalisVenator can probably give you some help with Raam (light emergence is probably the biggest thing here though, as it should punish all his reckless air approach options). Gargos’ st.HK can fulfill that same role for him, though I’m not really sure how the rest of the fight shakes out.
And finally, I did make a video a while back talking about dealing with Rash as Hisako. It won’t all be applicable, but hopefully some of the stuff (particularly the way I challenge his tongue approaches) can be helpful to you. 
Rash is a natural hard counter to kan ra, also has the advantage on gargos and arbiter. Glacius and raam deal with him fine. Any time he tongues most characters can actually punish him with a medium button unless he angles it right, which is some thing your typical rash player won’t know how to do. In glacius case you can just dp him after either.
Rash actually makes a huge mistake if he tries to approach raam too hard. Raam will tear him apart on knockdown and st. Mp can punish tongue approaches. Cr. Hp, medium emergence or kryll shield punishes wrecking ball.
With Arbiter you just have to anti-air wrecking ball with df.HK, and punish tongues on the ground with st.LK > mixup. If you have overshield then his tongue won’t put you in stun as long as you don’t try to block it, so you can sort of walk him down and play lame. Unless the Rash player knows how to use his normals well you pretty much have nothing to worry about with shield on.
What does “df.” mean?
I assume it’s “down forward.”
How I win against Rash is by being an annoyIng opponent as Glacius. I always anticipate that Rash will do that ■■■■ wrecking ball a lot so I jump kick with a Heavy as much as he attempts a wrecking ball. Also, hit him with repeated pokes, maybe once or twice, then surprise him with a throw. When he realizes you start throwing after pokes, don’t throw but jump, if he throws he will be open to more pokes. This is very effective when you can trap him in a corner. Mix up pokes and throws. Also you can puddle under him and poke him from the other side. Examples: poke, poke, throw, crouch poke poke poke, throw, poke, jump, poke, jump, poke, throw, puddle under to opppsite side, poke, poke. I’ve won many rounds like this.