Lucario is at a disadvantage to ROB.
You can't gimp ROB. You need to flat out kill him. While ROB is fodder for many, many of Lucario's combos, things like ROB's FTilt/FAir/DSmash/etc, are all very powerful, quick, and have range, and stand to interrupt much of Lucario's flow.
ROBs will get you over the ledge and keep you from returning, so fighting him from the stage is the best bet until you take control of the match.
If ROB is approaching with a SHAerial, Lucario's best option is to shield whatever it is and punish it as soon as it ends.
If ROB uses NAir, shield it and roll away from ROB to get out of DSmash range, and avoid a possible grab, both of which can be punished with a step forward and an FSmash, or better yet, an aura sphere.
If it is FAir, you can just shield grab or whatever you fancy out of a shield.
If it is a BAir, do what you would do for FAir.
If it is a UAir, wtf?
If it is a DAir, punish that crap.
When ROB is pushed to a ledge, he'll more than likely ledge camp with Tops/Lasers/NAirs from the ledge, and drop down and grab it again.
The best way to combat this is to fake him out and DAir him. Trying to approach him from the ground will get you attacked more often than not, and at least using DAir you can slow yourself down as you approach.
If you DI downwards and to the side after any of ROBs throws, you should be safe from a majority of his follow ups, most notably his laser.
If you're both at low percents and he fires his top, go ahead and grab it before you start chaining Force Palms on him, so he can't charge another one after he escapes. You can chain him for a relatively long time too.
ROB still outlives Lucario though, unless Lucario is cranking 150% and gets in a Tippered FSmash on the ledge, you're looking at ROB living past 140% per stock.
Best stage for Lucario is Luigi's Mansion or Norfair against ROB. The best neutral is probably Yoshi's Story. Its about racking up damage and staying alive, right? Sounds good to me.
-DD