I have this Riolu in which it has good IVs of
31 SP. A
31 SP. D
Copycat -- To copy sweepers and fly skrubs
Dragon Pulse -- Bye-bye dragons
Earthquake -- Coverage: kills rock, fire, steels and poisons!
Brick Break -- Annihilates pesky reflects and light screens: Better safe then sorry :D
so.. what should I train it in (It will be a mega Lucario)?
For this particular set I would recommend 252 Atk / 4SpA / 252 Spe. Two of your three attacks are physical, so pumping those up as much as possible is a good thing to do. If you want to kill something specific with Dragon Pulse you could delegate a few more EVs into Sp. Attack, but if not then 4 EVs is good. Because of your middling Speed IV — and I would highly recommend breeding for a max speed Riolu, because Lucario is so flimsy it needs to be as fast as possible — you want maximum EVs in Speed. Of course, every Lucario wants to have maximum speed anyways.
One conflict I see here is running Dragon Pulse on a Jolly set. This is contradictory, because Jolly lowers your Sp. Attack, as well as Dragon Pulse's effectiveness. If you have access to the Move Tutor in ORAS, either Ice Punch or Dual Chop would be better options. I suggest Ice Punch because it covers more types. If you go this route, take the 4 SpA EVs and put them in HP.
My Lucario set (physical)
252Atk 252Spd and 6 for whatever ev you want
Close Clombat/High Jump Kick