A couple of things to consider for in-game Battle Tower:
-Natural Cure and Aromatherapy/Heal Bell: You don't get Items and a good majority of the time you're not going to want to waste your limited pool of Hold Items on a Lum Berry (though if you have nothing better to use, it can't hurt). Having a Blissey or Miltank to heal your team or guys like Starmie and Blissey that heal out of status effects on their own is a major boon against annoyer sets, Tri-Ahax, and random Quick Claw hacktivations on a sleeper move.
-Sweeper: You want guys that will hit and not let your opponent's dude get up. Garchomp, Salamence, Metagross, Gengar, and Weavile are some of the more prominent ones where they'll generally outspeed anything your opponent sends out (except for Metagross, who is generally study enough to afford being slower some of the time).
-Wall: You need someone who can take hits like a king. I cannot put into words how many times I've swept/had to sweep using a Blissey or Bronzong in Battle Tower because a little bit of good luck/prediction by the computer would end my game. Self healing is a must.
-Resistances: You need to have as many resistances and preferably immunities as you can. Two big ones are Normal and Ground to resist Fissure and Horn Drill. Gengar is one of the best men you can have when it comes to beating OHKO moves, as he's only weak to Sheer Cold.
I used Starmie, Garchomp, and Blissey to accumulate 98 wins in Battle Tower and probably spent at least 100 hours trying to get my 100 streak to no avail. Prediction is important but so difficult because you never know which "model" of a Pokemon the computer will send out (is it the Curse/Dragon Dancing Physical Lapras or the Rest/Talk/Fissue/Sheer Cold with Quick Claw one?). Sometimes you may make a hasty decision to try and minimize the chances of the almost ridiculously accurate OHKO moves from killing you (ex: keeping Garchomp in on the Aqua Jet/Protect/Ice Shard Dewgong expecting Horn Drill/Sheer Cold to risk the OHKO by trying to OHKO it first so you don't chance losing a man on the switch).
For the most part, if you have a good wall like Blissey, the tower is really easy as you can usually ride the damage soaking capabilities of your wall to victory without losing a single man. This usually takes long, but patience is definitely a key to victory in BT.
