One of my training partners plays Ermac and I don't have too much trouble outside of his frame trap strings, you just have to respect them. I normally wait until he pokes or does something unsafe, which normally get thrown out to cover one of his strings.
He has a few options after his 11d2 (I don't know his full strings, but the string off 1 that is his fastest start up). It's 0 on block, so he can either do the string again, do the low option, go overhead, or poke.
If he does the string again, you can hopefully start abusing the gap that's between the second and third hit. You can either leg grab or parry this so you'll hopefully shy him off this after a few times.
If he does either the low or overhead you need to just armor after he does the string to check him. I do this maybe once every 10 or 15 times just to make him start poking (faster option) to check me after a string.
If he pokes you afterwards, he's insanely negative (think his d1 is -10?) so you almost never really need to poke against him. Once you get him to do an unsafe string or poke, just go into your mix-up. You should trade better from that point on and hopefully have a lot easier time opening him up than he does to you so you can trade full combos to his chip pressure (as above).
The next huge part of this game is bullying Ermac on wake up. He doesn't have great options on this, either EX soul blast you can normally get in on him again and his EX teleport is SUPER unsafe, so if you're wiling to block his wake up's you should be good. However you can NJP over all of his wake up options (unless you angles the soul blast upwards). You can jump backwards (not great because it slows pressure) over his tele for a full combo to put him in the corner or you can jump over him (to put yourself in the corner) to totally make the soul blast whiff as well.
Once Ermac is on his back he has a pretty hard time, but one thing my friend has been doing is after a combo ending in 21u4, I have to run up to him to continue offence so he will wake up and just do a faster string, which catches me.
I tend to just jump in on him a lot as his only semi-reliable anti-air is his uppercut (with a good forward hit box) or cross him up as much as you can.
Hope this helps
@Tony at Home