I'd be glad to help.
After many, many, many tries in Diamond, I got through with a hacked but legal team. I made it ti 130+ wins. I have not gotten past 100 since then. It is very much a challenge, and without doing researc on how the Battle Tower/Frontier works, it's also a lot of luck.
Remember the AI doesn't need to win every Pokemon battle, or even every match. They just have to beat you ONCe before you start over. They will use Double Team, Lax Incense, critical hits, OHKO moves, and all types of crazy stuff that is just flat out annoying.
So, before you build your team, you need to do fit in as much advantages as possible.
You need to have a competent understanding of EVs, IVS, natures, and all the other game mechanics that affect battles (physical/special split, statuses, etc...). One can MAYBE get through the first 21 matches by only EV training their Pokemon. After match #50, the AI starts using a ridiculous number of perfect 31 IV in all stat Pokemon. The only advantage YOU have is that their nature is random, so it might hinder them, boost them, or be neutral.
100% accuracy: While not all that common, you will eventually encounter a Double Team abuser or evasion inside hail/sand. You can view all 100% accuracy moves here. I usually ended up slapping Aerial Ace on my Band/Scarf Garchomp.
Status absorber: The AI will abuse stats to no end. However, status (as in poison, burn, sleep, and paralysis) can NOT be stacked. So if you had a Poison Heal Breloom with a Toxic Orb, then the AI's attempt at sleeping Breloom will fail. RestTalk sets (not a fan), or abilities that can handle status are also options. Individual Pokemon immunities will only cover so much. I like having one Pokemon absorb all status, and usually utilized Natural Cure on Starmie for that. Substitute sets are also a good enough check.
OHKO absorber: Sturdy means OHKO moves will fail. Unfortunately, there's only a handful of Pokemon with the Sturdy ability, and even fewer that are actually good. Donphan has great attack and defense, but sucks everywhere else and is soo slow. Magnezone suffers from the ground and fighting dual weakness (2 most common types) but has great special attack and decent bulk on both defense and sp def. Both, however, have very limited movepools and if you're playing completely legit, Magnezone won't be bale to hurt as much without HP Ice/Fire/Fight/Ground/Grass.
Substitute sets are also a poor-man's option here too, giving you time to prepare/KO something. Shedinja's Wonder Guard gives it an immunity to all OHKO attacks, but it's Shedinja, so yeah.
Minimize the need to set up: Even though I used a Swords Dance-Sub Scizor (requires set up) and many people swear by Swords Dance Garchomp, I encourage to minimize that as much as possible. You want to hit hard, hit fast, and be done. This is what the Choice Items were made for. Having two on a Battle Tower team is NOT a bad idea.
Cover your weaknesses.: Between your three Pokemon, when one has a weakness, another should resist or be immune to it. THIS IS THE MOST IMPORTANT. Switching is something you'll do often in the BT/BF and the AI won't (unless they can't hurt you at all). But too many attacks in this game, for neutral damage, can result in an easy 2HKO. So, just as an example, and without looking at the move set, here's my winning Battle Tower team:
Garchomp (Dragon/Ground): Weak to Dragon and Ice. Resists Fire, Rock, and Poison. Immune to Electric.
Scizor (Bug/Steel): Weak to Fire. Resists normal, grass, ice, Psychic, Bug, Ghost, Dark, Steel . Immune to Poison.
Starmie (Water/Psychic): Weak to electric, grass, ghost, dark, bug. Resists Fire, Water, Ice, Fighting, Psychic, and Steel.
Garchomp's weakness to dragon attacks is covered by Scizor. It's Ice weakness is covered by both.
Scizor's weakness to fire is covered by both (though in case of a burn, Starmie can recover while Garchomp would be stuck by having his attack cut in half).
Starmie's electric weakness is covered by an immunity from Garchomp. Scizor resists everything else that Starmie is weak against.
So wow, that's a lot to think about.
In fact, that's so much, that I'll just come back and rate your team later. Or hit me up on IRC. randomspot555 in all the project pokemon channels.