I think the way most people lose eventually in the battle tower is through getting bored with easy battles. They all seem so easy, then along comes the cpu opponent that gets mega hacks, and maybe you may make a small mistake due to intense boredom, and lose.
This team got me 314 wins, it lost to a regiice that got two quick claw boosts, a critical hit and an ancient power boost :(
Evs 252 speed 200 spc att 58 att
This guy basicly sets up rain dance and explodes. Unless his stabbed psychic is supereffective vs the pokemon im facing.
Evs 252 att 252 spec att 4 hp
This guy usually has rain dance set up for him and is a complete sweeping beast. Weather kicks ass in 3 on 3 battles, who needs damp rock? Cpu cant stall you out with intelligent predicted switching.
Garchomp (the uber it lets us pick)
Evs 252 att 252 speed 4 def
Fire fang was generally useless because i was usually in the rain. But i hate using low accuracy moves in battle tower like stone edge, you need to be consistent. Normally the pokemon you need fire fang for are tanks and slow to dish out the damage anyway, needing multiple turns to kill you.
Most battles garchomp never even needed to see play, kingdra owned that hard for me.
Yache berry guarantees a sword dance set up.
I think the key to winning in battle tower is having fast cheap pokemon and making good use of your items