Deoxys would be fine.. but... that's cheating!
I beat my Diamond (okay, you got me. Platinum's harder) with underleveled Pokemon.

And yes, I caught Feebas, Munchlax and Heracross by myself. No cheating. Feebas was the only one whom I really searched for. I stuck to Mt. Coronet for about four hours before catching a whole box of Feebas. =3 None of which liked Dry Poffins though. xDD
Munchlax and Heracross were by accidents since I just wanted to finish all the Honey in my bag. xDD

--
"Everything's an illusion. Your hopes, your friends, your life itself... shall fade." -- from Ultimecia
--
"If growing up means throwing my imagination away and act like boring people, then no! I refuse to grow up! I won't allow it! I'd rather die!"

Torterra needs Crunch and Earthquake! Level him to 42. That should be about the level I got Earthquake and Crunch.
Luxray... keep either Thunderfang or Spark. Take Tackle out... err... you can keep Bite. And... I got no idea. I never used Electric Pokemon before in an Elite Four challenge.
Gabite... turn it into a Garchomp like ASAP since it wouldn't last long when hit with a Water or Grass attack. Retain Dragon Claw, get a Ground attack like Earthquake, make it learn Crunch and the last move is up to you.
Staravia... umm... just no. I won't work. Crobat is much better. But... if you wanna use Staravia, you should retain Endeavor and Quick Attack. It works wonders. Google "FEAR Pokemon" and you'll know what I mean.
Houndoom... re-learn Thunderfang via Move Tutor, get Crunch, retain Fire Fang or replace it with Flamethrower and you may spam with Sludge Bomb, SOlarbeam or Shadow Ball. It's up to you.


What I think:
For Aaron, Houndoom pretty much kills everyone except for Heracross who can STone Edge or Earthquake it to death and Drapion can just X Scissors it or something. To make life easier, if you can get a Crobat, do it since it's way faster than Heracross and more useful than Staravia (sorry, Staravia really can't do much now). Drapion means Earthquake all the way from Torterra. Don't worry, Torterra should be faster than Drapion. If not, you got to level him up more then.
For Bertha, use Torterra on all except Gliscor since it will most definitely screw you with Ice Fang. Instead, if you plan to use a Blissey, now would be a perfect time to let it go stall-mania. Just remember to Softboiled every once in a while.
For Flint, Gabite can crush them all even Infernape if you haven't allowed them to use Sunny Day. If they do, you're screwed. Torterra's Earthquake should work here if they don't hit you first.
Lucian is by far the first of your worst nightmares considering your team. Whereas Houndoom can take out only two of his monsters. Alakazam will slaughter you with a Focus Blast, Bronzong will Earthquake you if you don't kill him in one hit and Gallade has Drain Punch and Stone Edge to beat your Houndoom to dust. I recommend a Drapion to beat the crap out of all of them except Bronzong. Blissey won't be able to help much here but Counter will save you from instant Earthquake or Gyro Ball death.
Cynthia, (holy fruppe), would be the worst. Blissey will very much save you from Spiritomb, considering both Toxic and Sweet Kiss. If you must defeat it with your offensive Pokemon, try and take it down with one attack. Spiritomb really hurts your PP. Roserade is a joke. Don't worry, Flamethrower will easily kill it. Togekiss must be defeated by all means. Although it's not so powerful as her other monsters, its Aura Sphere really hurts. And it's not even STAB! Lucario will pretty much kick all your Pokemon to dust. Your best be would be kll ii if it doesn't kill you first. Torterra may survive its Aura Sphere. So try! Milotic will pretty much be a very bad sight for you now. So... don't attack it with a Special Attack because it will Mirror Coat it back to you if it survives. Just don't! Hit it hard with Spark or any other physical attacks. Don't use Torterra! It will Ice Beam the poor turtle. And don't use Garchomp unless you are sure you will beat it first. As for her Garchomp... kill it quickly! If it uses Giga Impact and faints one of your Pokemon, put your strongest surviving Pokemon up next and do what you must!


'Kay, that is it. Here's a list of available Pokemon which you can add to your team if you have the time to train (these are what I recommend. you don't need to oblige though. xD):
1) Blissey (obviously. X3)
2) Crobat (faster than most Pokemon of the Elite Four. It even beats Garchomp!)
3) Bastiodon (if you still favor stalling. X3)
4) Heracross (this one would pretty much slaughter all of Cynthia's Pokemon if trained right. Ideal Moveset for such slaughtering is Aerial Ace, Close Combat, Night Slash, Stone Edge. You only need to use 1 TM which is STone Edge.)
5) Bronzong (a very good staller and attacker. xD)
6) Spiritomb (like Bronzong... only better because it makes opponents Struggle faster. Thank you, Pressure. xD)
7) Sudowoodo (good defenses, awful speed. But Sucker Punch is very, very painful! Even if it's not STAB!)
8) Lucario (very awesome Special Attack or Attack)
9) Mamoswine (would be good)
10) Weavile (like Crobat. very fast!)
11) Toxicroak (painful... just painful. Sucker Punch really bites on Lucian's Pokemon. Poison Jab is just darn awesome.)
12) Drapion (a Pokemon with good offense and a very good Defense... not Special Defense but no Special Attack can really hurt Drapion.)
13) Clefable (another stalling monster although not as good as Blissey...)
14) Snorlax (yes! Even Snorlax can help you!)
15) Tentacruel (a fast Water Pokemon... and it's half-Poison to boot so Solarbeam won't KO you.)
16) Milotic (Why would Cynthia and Wallace use her if she wasn't so good? -_-)
17) Mantine (a defensive Water Pokemon)
18) Gardevoir (another Psychic... not as good as Alakazam though)
19) Alakazam (It was once a very overpowered Pokemon. Seriously. Back in the days of RBY...)
20) Dusknoir (like Spiritomb. x3)
21) Magneton or Magnezone (better ELectric monster... although it would die at Bertha. DX)
22) Absol or Murkrow or Honchkrow (Super Luck will help you kill them easily)




That's all for now... whew... that typing made me tired. xDDDD

--
"Everything's an illusion. Your hopes, your friends, your life itself... shall fade." -- from Ultimecia
--
"If growing up means throwing my imagination away and act like boring people, then no! I refuse to grow up! I won't allow it! I'd rather die!"