Try to put him in a position where his only option is to teleport. Bait the teleport out, make him think you're coming to him, when he tries to teleport, block and punish, if you think you won't land a combo, or you crack, just hit an uppercut. Close distance with elbow, if he tries to teleport behind you, or you think he will try to, get him with a combo, and he if blocks it, try to dash before he teleports, and cancel the dash with block. You just have to read the ones who can teleport. I have more problems with Cyrax than Ermac any day. But, I can see why it's difficult. All match ups take practice, due to Reptile, with the exception of Dash Elbow, being a bit less mobile than others, and having a bit of down time on his combos, you just have to plan your moves a few steps in advance, and make good reads.
In my experience, playing my friend who's got a pretty legit Ermac, the match can go either way. If he keeps you at his distance, and keeps tagging you on messed up Forceballs, dash elbows, or slides, it's his game. If you don't let him get distance, and actually get close to him, not a lot he can do. If you keep the pressure on with combos, and he tries to teleport away, block, and punish hard.