Thundershock is really the way to go on this one, and I agree with spyckie in that it is an offensive tactic, not a defensive one. The point isn't to just hang back thundershocking - it's to open him up to a quick well... anything. Thundershocks are just so great because they can be used for either an approach or a retreat, against most of the cast. I often turn my retreats into approaches, and they're just in general really great for gaining the upperhand in this matchup.
CTs are gamebreaking, and typically you're going to want to work toward either a gimp offstage using thundershock/nair/fair (I call anything less than 100% nowadays a gimp lol), or setting up thunders using your smashes after whiffed attacks or just sneaking in using your speed.
The fsmash is actually a huge friend of mine in this matchup, in addition to the staple dsmash and running usmash. The fsmash's range is nice for punishing whiffed hits or just plain breaking off his approach, and often can help lead into the aforementioned edgeguard or thunder.
Also, work on smash DIing his AAA combo - that used to be one of the biggest killers for me when I played ike, due to its speed and raw power. Ike works a lot on punishing mistakes, but if you take away his quickest kill move, you force him to make some riskier moves, allowing you to turn his own game around on him.