Hmm well i know Ermac Quite well since iv been playing him for a long time, and the Ermac Community here is quite helpfull so we all lvl up at quite a good rate. Well I would have to be in your shoes, playing the dude to see exacly whats happening, but from experience of seeing other online Ermacs, let me guess, he closes the distance and does D3 couple times then goes for Overhead or low right? If thats the case you can poke out if you block the first D3/D1... If he is rushing you and going straight for the 50/50, again you can poke out before any of the mix ups come out, try to check him with D1 or 11...
As for the other options you mention, ppl are never complitely random, they have a pattern, always, you just gotta find his... 222 Bf2 is full combo punishable so if you are getting pressured, try doing D1 to poke out and then go for B124 you can backdash safely from that string, even 222 cant catch you from that... Optimally he will only be able to attack you from 2 places, a Jump in, or running in for a string... If he tries to Jump in your best bet is B1 AA into a full combo, but this requires you to have really good spacing and footsies From the ground you just gotta play Footsies with him, B124 is your friend, use it, it ha snice frames. On some opponents you can check them with 11 and then go for a 22, try to be unpredictable with not finishing your strings complitely so that they are wainting for the last hit of the string and then you just 50/50 him cause he wont be able to react in time. EX: go for B12 50/50.. or do 111 checks, just keep in mind 111 string, the first hit is a high. If you dont know what 111 checks are, its basicly a fast string that will catch any opponent trying to press any button that is not a poke... so basicly its doing 11 pausing for a frame doing another 1 or 11 and then going for a mix up. If you see he wants to press buttons inbetween your 11 poking, just finish the string 111 and it will catch him into a KD...