Ash is not an easy character to start the game off with, but if you're used to charge character play, most of the time you won't have to do much. In fact, if you learn how fast your moves are and what you punish, you'll be on easy street. Take for example when K' gets you in the corner and he does Ein Trigger => Second Shoot (F+D, the high kick follow up). He's not safe. You can punish him royally.
If K' does Minute spike by itself. That's also not safe, get in a full combo. If he does Narrow Spike, also not safe, you should definitely punish that. Almost all his jump-ins have to be landed meaty to hit crouching people so a poorly timed hit can mean full combo. People like Ash can do low B to lower their hit box and catch them when they whiff.
Now, aside from his EX moves, which are a little harder to catch, his neutral game is a bit easier to deal with and gives you ample opportunity to beat him. You just have to be patient.