From what you've given us, this is the best team that I can come up with.
Blaziken L63 (Sky Uppercut, Rock Tomb, Aerial Ace, Blaze Kick)
This is obviously the ace of the team. Sky Uppercut can easily OHKO either Sydney's (first E4 member) Dark Team or Glacia's Ice Team (third E4 member). Blaze Kick can also easily OHKO Glacia's Glalies as well. Both of these moves also work well against Steven if he's the Champion (if you are either playing Ruby or Sapphire).
You should replace Slash and Strength with Rock Tomb and Aerial Ace (teach via TM) for type coverage. These moves would be particularly helpful if you are playing Emerald and Wallace is the Champion. Aerial Ace for Ludicolo and Rock Tomb for Gyarados.
This should be the first one upfront when your brother faces this first Elite 4 member.
Flygon L55 (Crunch, Dragon Claw, Fly, Earthquake)
A good, solid Pokemon to use.
Crunch can help against Elite 4 Phoebe (second E4 member) and her Ghost types.
Similarly, STAB Dragon Claw can help against Drake (last member before Champion).
Fly and Stab Earthquake are good for type coverage. Earthquake especially if the Champion is Steven. If the Champion is Wallace, Earthquake is particularly useful against Tentacruel.
Breloom L55 (Toxic, Bulk Up, Mach Punch, Leech Seed) or (Sunny Day, Bulk Up, Mach Punch, Solar Beam)
Teach Bulk Up and Toxic to Breloom.
Bulk up raises its Attack and Defense stats by one stage each type its used and with Mach Punch's priority, this combo can be quite useful against either Sydney or Glacia.
Since 3 of Glacia's 5 Pokemon are all half Water types, your brother can use the Leech Seed/ Toxic combo to whittle away at her team's health while you regain any health lost.
If Wallace is the Champion though (if you're playing Emerald), the I would suggest teaching the Breloom the Sunny Day/ Solar Beam Combo. It would weaken water type moves and OHKO his pure water and Water/ Ground types with an instant Solarbeam.
Whiscash L55 (Earthquake, Ice Beam, Surf, Waterfall)
Decent Pokemon.
STAB Earthquake useful against Steven's Steel and Rock Types or Wallace's Tentacruel. Ice Beam can be useful to combat all of Drake's Dragon Types. Surf and Walterfall are Powerful STAB moves against any Pokemon either weak or neutral against Water types.
Walrein L55 (Hail, Blizzard, Body Slam, Rest)
Evolve Spheal to it's final stage. This is your brother's ace against Drake.
Hail/ Blizzard Combo easily take care of Drake's Dragon types (Blizzard 100% hit in the Hail), just make sure to use 3 PP Ups to maximize Blizzard's use. Rest for instant healing (use full Heal Awakening or make it hold a Chest Berry) and Boby Slam is a strong, netral move that has the secondary effect of causing paralysis.
Absol L55 (Double Team, Swords Dance, Shadow Ball, Slash)
Catch an Absol near Fortree City. This will be your brother's ace against Phoebe.
Teach it Shadow Ball and it learns the 3 other moves naturally.
Firstly, spam Double Team to increase evasion. Then, spam Sword Dance to maximize Absol's already ridiculously high attack stat. After that, keep spamming super effective Shadow Balls to take out her Ghost types.
Keep Blaziken where it's at level-wise, but train everyone else up to level 55. Not too over or under leveled.
If you need to learn anything more about each of these Pokemons' movesets or where to find certain TMs, you can look it up on Bulbapedia if you wish.
I hope this helps!!! :)