@ Choice Scarf
252 HP / 236 Def / 20 SpDef
~ Thunder Wave
And so it begins. I chose Latias to be my Lead because of not only her great Defences, but also because of her ideal movepool for a situation like this.
With a TrickScarf lead, it is important to be able to take multiple hits from a range of opponents, and Latias is certainly capable of doing so. The given EVs allow for well-rounded defences and great Bulk, giving her 187 HP , 154 Def, and 153 SpDef.
There is no Pokémon in the Battle Tower that Latias will fail to pass her Scarf to. Not even Scarfed opponents that land a Super Effective Critical Hit can OHKO, and in most cases she will also be able to hit the opponent with a Thunder Wave to cripple them and negate the Speed boost from that Scarf. After this, she will still be alive to Recover damage and proceed to spam Flash until it is a perfect time to bring in another team member.
252 HP / 252 SpAtk / 4 SpDef
~ Flash Cannon
~ Calm Mind
~ Iron Defence
In many other circumstances, this Lucario would be destroyed before it could have the chance to set up. But thanks to Latias, it is far easier for Lucario to switch in and set up once the opponent has been completely crippled.
The EV Spread makes Lucario bulky enough to take a fair beating after a setup and still survive, but it also gives it a great amount of firepower behind Flash Cannon. Max SpAtk is used because Lucario will need all the power it can get to deal with heavy resistors and bulky waters, as Flash Cannon is its only attacking move.
Having only one attacking move does not worry me, however, as with a good setup from Calm Mind, it will be hitting at insane levels that would rip through even the bulkiest Empoleon within a few hits. As soon as Lucario comes in, it will get a Substitute up and use either Calm Mind or Iron Defence first, depending on the attacker. Once the setup is done, it will proceed to Sweep, and hiding behind a Substitute, it does not fear Critical Hits or OHKO moves.
@ Chesto Berry
252 HP / 4 Def / 252 SpDef
Last, but not least, Swampert. I chose to have Swampert on my team because of its great bulk and brilliant typing; Water and Ground has only one weakness, Grass, which Lucario or Latias, if she is still alive, can take hits from. With its bulk, Swampert can come in on a variety of opponents that might otherwise cause Lucario trouble and set up.
Swampert's EVs allow it to take hits better from the Special side of the attacking Spectrum, as that is where it cannot boost its own defences. The Chesto berry allows Swampert to get an immediate heal when it needs one, and also to cure itself of Burn or Poison that it may have picked up.
Together, Return and Waterfall provide at least standard coverage on almost all opponents, only hitting for "Not Very Effective" damage on Empoleon. Curse is in there for the obvious reason; to allow Swampert to tank effectively and take out anything in its way.