Uxie Mesprit @Leftovers Choice Scarf
Ability: Levitate
Nature: Bold
EV: 252 HP, 6 Def, 252 Speed
~ Thunder Wave
~ Flash
~ Substitute Trick
~ Light Screen Charm
IVs are important in the Battle Tower, so please consider getting high values for those. There are very few Pokemon that can use this set in the Battle Tower. Mesprit is one of them. As stated earlier, Latias is another. Uxie has much higher defensive power, but it cannot learn Charm. As someone already stated, you Trick first turn, and with Choice Scarf equipped, you outpace almost every other lead. Then proceed to Thunder Wave. If the opponent is a ground type, then start with Charm or Flash depending on what it uses first turn. Because you locked your opponent into using one move, there are a few moves to consider. Notably: Toxic and Thunder Wave. Thunder Wave is useless against your Electivire and Garchomp. However, Toxic can ruin your day because you have no one immune to Toxic.
Your Substitute and Light Screen are not very effective. Substitute doesn't help Uxie or Mesprit much. There is only so much you can do with Thunder Wave, Flash, and Light Screen. And once you have a few of those moves in, you will be stalling with Substitute because you can't use many other moves. Also, your moveset is not useful against Metagross. You can Thunder Wave and set up Substitute and Light Screen, but that's all that you can do. You also wasted that Substitute if you use it.
Electivire @Expert Belt
Ability: Motor Drive
Nature: Adamant
EV: 252 Attack, 6 Def, 252 Speed
~ Thunderpunch
~ Ice Punch
~ Cross Chop
~ Earthquake
As stated already, Electivire is not useful in this team. He is very much outclassed by Garchomp in every aspect. Good coverage with moves, but he just can't dish out attacks very hard. He also doesn't make good use of your Lead. Your lead's job is to buy you time to power up so you can OHKO your opponent. Your lead isn't going to Thunder Wave, Trick, and Charm every opponent. So you can't just plain out attack and hope for the best. This is the Battle Tower. Your opponent's Pokemon are high leveled with good moves and some have perfect IV. And I'm sure that they are EV'd as well.
There have been other suggestions. Test them out. The key thing is that you need a Pokemon that is immune to Toxic and can set up. It is NOT important to have a special sweeper here!
Garchomp @Lum Berry
Ability: Sand Veil
Nature: Adamant Jolly
EV: 252 Attack, 6 Def, 252 Speed
~ Fire Fang Substitute
~ Dragon Claw Outrage
~ Earthquake
~ Stone Edge / Swords Dance
Immune to Thunder Wave is great. Swords Dance to power up under Substitute if you so choose. Outrage is the better choice here. Alongside Lum Berry, it can be used twice. And if you Swords Dance enough, that's game.
I'll leave you with this:
The Battle Tower isn't a drawn out match with 6 vs 6. It's 3 vs 3. That calls for a little different style of play. And every Pokemon you choose with its moveset is critical. A loss of any of your Pokemon can result in you losing and starting all over again. Think of Chess with only 3 pieces (one of which is the King or in this case Lead). So you're down to 2 slots to make good use of. If you make a bad move, you're done! End of story.