Not a bad team. I'd replace Abomasnow though. I wish you had posted what moves they know tough! lol. Get them all at LEAST past level 60. I'll go over each of her pokemon and the best way to defeat them based on your team.
I'm willing to bet that the first pokemon she sent out against you was more than likely her Spiritomb, which should be on level 58. Its moveset consists of Silver Wind, Dark Pulse, Shadow Ball, and Psychic. This will probably be the most difficult one to defeat (possibly even more challenging than Garchomp), since it has no type disadvantage. Since it does not have a type advantage, and its dangerously offensive oriented moveset, you'll need to take it down ASAP. Hit it HARD! If I had your team, I'd use either your Garchomp or your Luxray (Garchomp, preferably). I chose those two strictly based on what you told us in your question, because like I mentioned earlier, you didn't post what moves they know, and that would've helped a LOT. Anyway, just hit Spiritomb as hard as you can.
Her level 58 Roserade knows Energy Ball, Sludge Bomb, Toxic, and Extrasensory. Once again, I don't know your Pokemon's moves, so my advice is based SOLELY on your Pokemon's type and her pokemon's stats. I'd use ice type attacks. If you have any physical oriented flying, fire, or ice type attacks, go with those. Roserade's defense sucks, but its special defense is above average. So hit it with a physical attack and defeat it in one hit before Roserade can use Toxic.
Cynthia's level 58 Milotic is a nasty piece of work. It knows Ice Beam, Mirror Coat, Dragon Pulse, and Surf. You're gonna have to take a hit or two from it before defeating it. The obvious choice would be Luxray. I hope for your sake it knows Thunder Fang. The move to watch is Mirror Coat. With Milotic's Mirror Coat, any special attack you send it's way is sent back at you with double the power. So if Luxray knows Thunder or Thunderbolt, do NOT use it. Luxray's special attack is not surprisingly good, and Milotic has a pretty good special defense. So firing a Thunderbolt it's way may not defeat it in one shot. Then, on top of that, if it used Mirror Coat, that attack is sent back with double the strength, potentially knocking out your Luxray. Now Luxray's attack power is surprisingly high for an electric type. That is why I recommend Thunder Fang. It stands a better chance of doing more damage to Milotic, plus Mirror Coat will not work against it since Thunder Fang is a physically oriented attack, NOT a special attack.
Cynthia's level 60 Lucario knows Aura Sphere, ExtremeSpeed, Shadow Ball, and Stone Edge. Use Torterra or Garchomp's ground type attacks, or Toxicroak's fighting type moves and defeat it quick, because Lucario can hit pretty hard. A fire type attack works too. Don't be surprised if Lucario moves before you.
The level 60 Togekiss has a nasty moveset that consists of Air Slash, Shock Wave, Aura Sphere, and Water Pulse. I recommend Luxray again, or if you have a rock type move, that is even better. You may have trouble. Togekiss' special attack is rather high, and all four of its moves are based on the special attack stat! It also has a high special defense, so focus on physical attacks.
Last, but certainly not least, is Cynthia's strongest and signature Pokemon, her level 62 Garchomp. It has Dragon Rush, Flamethrower, Giga Impact, and Earthquake at it's disposal. If you haven't noticed during my summary, your Abomasnow won't really help much during this battle, except MAYBE during your battle with Roserade. So honestly, I'd ditch Abomasnow (unless of course, you EV trained it) for another ice type, or a water type pokemon with an ice type move. But it's your decision. You will need an ice type move here no doubt about it. But why not Abomasnow? Garchomp is faster, and its Flamethrower will take it out in one hit: guaranteed. I'd teach a water pokemon an ice type move to use against it. Your Toxicroak could poison it with its poison attacks, but if you decide on this, you'll have to poison it again if Cynthia uses a Full Restore on her Garchomp, not to mention it'll have a Sitrus Berry. So overall, use ice type attacks. But not Abomasnow. I HIGHLY advise you to replace him. Sure, you had no problem battling with him before, but this is Cynthia now! Lol. Be prepared to use a lot of healing items. Use the turn Garchomp has to rest after using Giga Impact to your advantage since it can't attack.
So there you have it. To level your pokemon up the fastest, just keep battling the Elite Four constantly. Sure, you'll lose a few more times, but your pokemon will still keep the experience points they gained. Just a fair warning: you'll lose a chunk of money doing this though! Toxicroak could be replaced too, but it's not that necessary. Replacing Abomasnow is a must in my opinion.