If they turtle up, get to a comfortable distance, use Ermac's projectile for some long ranged chip damage. Be wary of teleports, if they toss one out, be ready to block it and start a combo off with his downback.X.
If they start tossing out their own projectiles too frequently, use his teleport, crouching jab, levitate to do some damage while they recover. Also mix up the teleport pattern, jump towards them, bait an uppercut, then teleport and land a hit.
Easy combo's include levitate followed by the choke move, or levitate, jumpkick, teleport, dash Y,Y, choke.
You can also land his X-Ray a second after the levitate hits, best to keep it to combo's and not waste it.
I find his choke is a good anti air from most distances, it's also very quick and can be used to counterattack fast specials like Liu Kang's dragon kick. Forward B is a nice command normal that hits them low while walking forward and if you are close to them after a knockdown mix it up with sweeps and throws.
I'm still new with Ermac, but these basics seem like more than enough for most people.