There's precious few Rock types in the game, and even fewer that are actually good. Your first Rock types are available in the Glittering Cave, so I'll skip all gameplay up until that point.
Gym Leaders
In the Glittering Cave, you can get Onix, Rhyhorn, Lunatone, Solrock, Dwebble, and Tyrunt/Amaura. Choose Tyrunt over Amaura, as it's stronger, has fewer weaknesses, and better coverage. Try to get three Pokemon here, taking at the very least a Rhyhorn, Tyrunt, and Dwebble. Onix, Solrock, and Lunatone are decent, but shouldn't be as prioritized due to their relative worthlessness in comparison to the other three. Make sure to grab a Binacle once you get on the path to Cyllage, as it's Water typing will be an asset against Ground types.
Grant should be relatively easy: Amaura falls to your STAB, and Tyrunt will go down due to being neutral to Rock STAB. Keep going once you get your badge, and you'll be good up until Geosenge, when you have to fight two Lucario.
Note: You can pick up a Nosepass before entering Geosenge. It's not very good, but it's nice fodder if you need to heal.
Korrina's Lucario have Fighting STAB In Power-Up Punch, so you'll need to overlevel Rhyhorn a bit. When it gets to Level 30, it'll be very physically bulky, and it'll have Bulldoze. If you don't win the first time, keep going until you win.
The entirety of Shalour is pretty rough for Rock types. Just try to use your overlevelled Rhyhorn for most of the Fighting Gym, with Dwebble in backup for the Fighting neutrality. Struggle your way through the Gym and the Tower of Mastery, and head onwards.
Ramos, while he may seem threatening, more than likely won't be. Keep your Binacle/Barbaracle and Rhyhorn away, and let Dwebble do the work. Rock Slide nails Jumpluff, while Bug Bite does super effective damage to Gogoat. Weepinbell may not fall to Dwebble, but it'll probably have a chunk taken out of it. In theory, Tyrunt's large Attack stat + Dragon Tail + consistent use of Revives should get you a win.
Proceed to Gym 5, Clemont's Electric Gym. Let Rhyhorn do the work as you smash Pokemon after Pokemon with Bulldoze, including would-be threats like Magneton. Clemont should pose no problem, and you can keep going forward.
You can manage to get a Rhydon before going to Valerie, but only just if you play normally. It's a great choice against her Mawile, and does a decent chunk to the other two members of her team. Crustle is also reliable to pick up the match should Rhydon fall.
The PokeBall Factory is easy, just like everything Team Flare. Smash it and keep going, using your coverage moves if powerful Rock STAB doesn't cut it.
Gym 7 should be pretty easy, all you have to do is get your Tyrantrum to a decent level. It's not very specially defensive, but it learns Crunch, automatically making it the best Pokemon for the job. Crustle, again, is an excellent choice to pick up the slack if Tyrantrum needs help. You could try Barbaracle as defense against Slowking's Water moves, but it probably won't be needed.
The Team Flare Hideout is, for the most part, a pushover. It's not too difficult, and you can use the free bed healing option whenever you need to. Lysandre is the only one that might pose problems, with Steel Wing Honchkrow and a Gyarados. Both die to good Rock moves though, and Mega Gyarados doesn't appreciate Crustle (who, you may have determined, is one of the MVPs of Kalos Rock teams) and its Bug STAB.
Wulfric is an Ice type master, giving your team it's easiest match since Grant. Abomasnow might be an issue, but it's slow and weak to Crustle's Bug STAB, plus all the other Rock moves you're sure to have. Squish him and head to the Pokemon League.
The League
Malva will not be hard. Annihilate her. The other three will be more difficult, but don't forget you're playing a Kalos game. They're still easy. Wikstrom is pretty easy to deal with with Rhydon's Ground moves and bulk, but mind the Steel weakness. Barbaracle also enjoys some time in the limelight, as it's neutral to Steel and has some pretty solid moves. Drasna's Dragon types get eviscerated by Tyrantrum's Dragon Claw, but be mindful that Tyrantrum is also weak to her STAB. Siebold will be annoying, but both Tyrantrum and Barbaracle resist Water. Make sure to not get tripped up by Clawitzer's Aura Sphere or Gyarados' Earthquake, and you'll be good to go.
Diantha may be troublesome, as she leads with Hawlucha. However, your team should be neutral to Flying Press, and thus it isn't a problem. Goodra and Tyrantrum commit die when faced with your Tyrantrum's Dragon Claw. Aurorus won't appreciate taking a Rhydon's Drill Run / Earthquake. Gourgeist suffers from Tyrantrum (with Crunch) and Crustle (with Bug moves). Gardevoir takes a lot from a solid neutral hit, so just attack it sit everything you've got.
Rivals
The rivals are so weak they aren't worth talking about. Meowstic dies to Crustle, Delphox dies to Rock STAB, and everything else dies to coverage.
Overall, a Rock type challenge will be tricky, but not impossible. Your team should have a Tyrantrum, Crustle, Rhydon, and Barbaracle on it. All are amazing attackers, and their resistances help against some of Rock's weaknesses. You may also use a Nosepass/Probopass as death fodder, or Onix/Solrock/Lunatone in the early stages after the Glittering Cave.
The opponents will be tricky, as Rock has a lot of weaknesses. However, it's also a very good offensive type, and Kalos are easy games. Good luck if you decide to attempt this challenge!
