When approaching ROB u need to keep a low profile, run and shield but most importantly duck. Kirby's horizontal profile when ducking is so low that it can piss some people off. Ducking lets u get under forward tilt and all u have to do is shield or spot dodge ROB's down tilt and forward smash.
Approaching in the air can be annoying. The Key to an aerial approach is patience and in my opinion a well timed neutral B then spit. Neutral B does 10-6 damage depending on how much u use it and it opens an opportunity to use ur forward our up air.
Positioning: ROB is weakest when ur behind him, ESPECIALLY when ROB is in the air. If ur being guarded by forward air wait till the ROB starts forward and do a jump+air dodge. U'll get above and behind the ROB. This is ur best shot at a hammer kill or Bair kill.
Gimping ROB: Down air doesn't work against ROB, he simply counters with Up air, instead u should go for a stage assisted kill. Not sure if u've seen My Combo Video, heres a link
http://www.youtube.com/watch?v=_EWKyT37KiY at the end of the video is a section on stage assisted kills. basically after inhaling u spit ur opponent out under the stage (Timing is key as soon as ur spit out puts them under the stage do it). After spitting the out u need to quickly jump to their height and Uair, Bair, or Up B, the move u perform depends on how the ROB Di'd
Misc. : If ur opponent spams Gyro, grab his gyro and hold on to it. ROB wont be able to use Gyro and will have to come after u. An aggressive ROB is easy to shield or counter. If u do throw the gyro make sure u REALLY try to catch it again, it will save u tons of damage and a lot of headache.
Hope this helps u out!
~Gonzo~