Hello,
happy to see people are still playing emerald :)
I played a bunch of Battle Frontier too, and these are my conclusions:
IVs are extremely important. IA rarely invests in speed and most pokemon in the Battle Frontier are bulky waters and normal types.
The best trio to complete the battle frontier is the following:
Metagross @ Choice Band
Ability: Clear Body
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Meteor Mash
- Earthquake
- Explosion
- Shadow Ball /Brick Break/Rock Slide
Banded Meteor Mash 2HKO every mon in the battle frontier, I wonder if someone actually managed to get the gold symbolds without him.
Earthquake is useful against the bulky waters that are very commun, and explosion takes care of an annoying mon you need to get rid of immediately (looking at you snorlax) or mons that can carry counter such as swampert and milktank. The last move will be a more accurate move than MM to kill either psychic, flying, or normal types.
For those playing on GBA, you can get a metagross adamant 27/25/29/11/28/31 38.80 sec after reseting the game.
Latios (M) @ Lum Berry
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Psychic
- Dragon Claw
- Thunderbolt
- Surf
Latios forms a good core with metagross, as they resist the other member's weaknesses. Latios has good speed and checks fire/ground types that can be annoying for metagross. Thunderbolt to deal with the bulky water types and skarmory, surf for the steel and fire types, especially houndoom. Since the AI doesn't invest at all in speed, modest nature is preferred.
Lum Berry is here because Soul Dew does
NOT work in the battle frontier, and we need leftovers for the last member of the team.
Encounter after 1 min and 16.20 second gives a modest 29/21/15/31/23/29 Latios.
Now this duo can usually win on its own, but bulky water types, houndoom and some ground types can be very annoying.
Swampert @ Leftovers
Ability: Torrent
EVs: 252 HP / 252 Atk / 4 Def
Brave Nature
- Earthquake
- Surf
- Ice Beam
- Protect
Swampert checks most pokemon in the game and should be kept for the mons mentionned above, as he has no recovery moves and can't stick around for too long. Avoid switching in swampert into an attack, as a critical hit or a SpeDef drop/burn will kill it in 2 turns. Protect is here for the passive recovery.
To get a good swampert you can use
smogon's breeding rng manipulation.
Seed 0:12.71 is easy to get and gives good ivs.
This team can do the battle frontier without too many problems. Hoewever there are some pokemon that deserve to be mentionned for specific areas.
Battle Factory:
This place is the best way to get introduced to the battle frontier. The fact that you don't need trained pokemons and items is nice. You also get to know the pokemon's sets in the Battle Frontier Area.
Taking set with wide powerful coverage moves is very good. A surprisingly effective choice was the special attack gyarados set. Double team are good but makes battles longer.
A good way to build is to only consider water/electric/ground types when building, as the other types can be dealt with much more easily.
Skip good pokemon with bad movesets.
Note: According to werster, the opponent's IVs in the battle factory are tied with your current win streak at the battle tower, making it easier when you have 0 wins there.
Battle Pyramid:
Making specific mons is really good in this area, especially for the trapping round:

Linoone
Ability: Pickup
- Covet
- Return
- Surf
- Thunderbolt
Very good to use on the 3 first rounds, as pickup works in the pyramid. Covet can also steal an item from a trainer, pickup working even if linoone is dead.
When you have linoone in your team, your goal is to get a quick choice band that will make your life 10 times easier.
You can get choice band as item/pickup on:
round 1: Floors 2 and 6
round 2: Floors 3 and 7
round 3: Floor 4
You can also get sacred ashes and leftovers on other floors.

Blissey (F)
Ability: Natural Cure
EVs: 4 HP / 252 Def / 252 SpD
Calm Nature
- Seismic Toss
- Aromatherapy
- Soft-Boiled
- Toxic
Blissey completly breaks the pyramid. Her ability to restore health and status to your other pokemons is amazing.She is good in all rounds besides 4,5 and 8. She also deals with zapdos and moltres against brandon which is always good. It requires RFLG and some breeding though.

Salamence
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Aerial Ace
- Earthquake
- Brick Break
- Rock Slide
Good mon if you get a second choice band. It can replace swampert after the first round and has a lot of pps to take care of the wild pokemons.
He doesn't have to struggle against the bulky water types that are everywhere on the other areas.

Heracross
Ability: Swarm
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Megahorn
- Brick Break
- Rock Slide
- Earthquake
Very good for round 6 and 7, not so much for the other. Another great user of choice band.
Taking time to get as many items as possible during the first 3 rounds is great. After round 4, avoid trainers at all costs, as they rarely give useful informations but can end your run (Curselax being the main threat). Note that a square of trainers can be formed around the exit areas, so check out for those.
Round 8 can waste a lot of potions and revives but you should be fine overall.
Battle Dome:
If you use Metagross/Latios/Swampert you will always be in the top left corner of the bracket. With good IVs you should win easily as Trainers have all 0 ivs from round 1 to 10. The average EV spread they use is mentionned in their desciption before the match.
A good mon to replace Swampert for is Breloom, as you just have to choose him when you know when he is faster than the opponent and heavily abuse spore+substitute. Since AI almost never invests speed EVs it is a really effective strategy, but it cannot be used as easily in other facilities.The maximum I got with him is 20 wins in a row, which is a tesimony of how effective it is.

Breloom @ Leftovers
Ability: Effect Spore
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Spore
- Substitute
- Focus Punch
- Leech Seed
There are no Jolly + good IVs before 9 min into the game so you will have to breed it. (see swampert)
Hyper beam/Return Banded Slacking is also a good option to finish the tournaments quickly.
Battle Tower:
Straightforward facility with the biggest downsides being that it has the biggest amounts of fights a.k.a chances to get haxed to death. Use the Metagross/Latios/Swampert core and hope for the best. On the last round, start Swampert, kill raikou, switch metagross vs latios and spam meteor mash.
Battle Arena:
Similar to Battle Tower but you have only 3 turn and you can't switch. Use the killer team and you won't suffer too much besides a potential houndoom.
You can use
this link for more information about the mecanics of the facility.
Battle Pike:
Avoid double battles/NPC rooms at all cost UNLESS you only have one pokemon left which unsures you to have an NPC room. Doubles battles can quickly get out of control with some bad luck. Other rooms are fine, though status can be risky. Take the trio and you should win.
See the differents rooms possible
Battle Palace:
Your pokemons choose themselves the attack they will use so spamming A and while watching a TV show might be a great idea for the 3 first rounds :)
In this area,
natures are extremely important, which is why you might want to replace your latios for another member with a hasty/naive nature. You can take another latios, salamence is slightly easier to get:

Salamence @ Lum Berry
Ability: Intimidate
EVs: 4 Atk / 252 SpA / 252 Spe
Hasty Nature
- Dragon Claw
- Fire Blast
- Hydro Pump
- Earthquake
This area being extremely random means that you will more than likely get few deaths before winning the gold symbol.
Note: remove explosion from metagross.
With this you should get 7 gold symbols without much issues.
Battle Frontier is a really fun and I really hope they would accept to bring it back in Gen 8. Pokemon players are getting older and there is definitly an public for this kind of post-game modes.