First off, throw Din's towards his feet. This is one of those super rare situations where jumping and approaching with Din's like that isn't too bad an idea. It stops gyro charging and it can easily trade hits with Robot, and really, outside of a fully charged gyro, I think you get the better end of the deal. Nayru's a telegraphed projectile (watch Robot for the start-up of the beam and just hope you can see the gyro in time).
Watch Robot carefully. Some are spotdodge/powershield -> Dsmash happy. Others are SH-bair and FH-nair happy. For Dsmash, it's just a matter of using Fsmash (sometimes a little charged and sometimes right off the bat to screw with spotdodge timing) and mixing it up with grabs. Basically, try Fsmash if you see a spotdodge coming, and try to grab if he's going to hold his shield. Dtilt is also a decent option, but his own is pretty fast as well, so be mindful.
Considering his size and how he moves, I don't have too many problems landing lightning kicks on him with short-hops and sometimes full hops. Watch out for all of his aerials, because as you probably already know, they're fast and/or powerful. You obviously do not want to be above him. If you're in his bair range, it's best to not even try. I believe his fair and your kicks will trade hits, but if you can get around that, you can set up a relatively easy kick. Overall, take him to the air when you feel that you can handle him; in my personal experience, Robot doesn't try to airdodge me or anything like that because players know how good his aerials are and how Zelda's are... but I use this to my advantage.
Definitely use U-something to kill for the most part. His weight and recovery really only place him up for edgeguarding if you Dsmash him at a % where he won't reliably die, and edgeguarding Robot with Zelda is hilariously terrible for Zelda. Din's can bring him back for extra damage, but if he's trying to use his Up B, go out and dair and/or footstool him. Some Robot players like to fire off projectiles at you while recovering. If they're above you using a beam, Nayru's. If they're below, jump + Nayru's (since hitting the bottom part of Nayru's will send it at least a bit closer to him than the top part would). Otherwise, wait patiently on stage by throwing Din's out and take any chance you get to gimp him.
Really, this fight is basically meant to be played out patiently and then stabbing him at certain opportunities. Even though Zelda is light, the only moves that can reliably finish you off are his more powerful aerials, Dsmash (if it isn't being used to death), Fsmash, his gyro, and a few of his throws. Again, finish him off with U-something (I like utilt the most for ROB, since uair is a bit situational and requires good prediction and Usmash is usually decayed, but Usmash if you're KO'd). Lightning kicks and Dsmash can also take him of course, but unless you're very skilled, you probably can't reliably land them in most situations to the point where they'd be your best option, and Dsmash is only good at moderately-high %s for Robot. Try your best to recover on the ledge or past him, and bait his smash attacks. It won't take you long to figure out how limited Robot can be once you've learned how to reliably counter his B moves with your own and when you start killing more with utilt.
