Alright I found out at work today that my boss was fired, which led to me having nothing to do at work. I had my DS which I usually play during breaks so I decided to spend my day getting paid $11 an hour to play pokemon! I did well enough to get on the leader board for both the HGSS Castle and Arcade in just one day.
Streaks-
Castle: 93
Arcade: 76
Hall: 128
Here is my team:

Porygon-Z @ Choice Specs or nothing (NN: ILOVEYOU)
IVs: 31 / 3 / 30 / 30 / 30 / 30
EVs: 6 Def / 252 SpA / 252 Spe
Nature: Timid
Ability: Download
~ Tri Attack
~ Hidden Power Fighting 70
~ Ice Beam
~ Thunderbolt
I went through many different options when deciding on the lead spot ranging from "disabling" Cresselia and Umbreon to Dragon Dance Salamence. The worst was battle 57-63 in the Castle when I experimented with Rotom-H, only to find it reverted form which wasted me a lot of CP. Porygon-Z ended up being the best option for me because of its outstanding special attack, multiplied with download hopefully and excellent coverage its moves gave. Despite this, ILOVEYOU did seem to be the weak link on the team because it gets killed easily and I am still experimenting on the lead spot.
Sidenote: I used this in the Battle Hall as well with Choice Scarf/Choice Band/Focus Sash and Dark Pulse over Ice Beam vs. Ghosts and got a streak of 138, losing to a snorlax.

Garchomp @ Choice Band or nothing (NN: none, it was traded to me)
IVs: 31 / 31 / 31 / 31 / 31 / 31
EVs: 4 HP / 252 Att / 252 Spe
Ability: Sand Veil
Nature: Jolly
~ Earthquake
~ Outrage
~ Dragon Claw
~ Stone Edge
Garchomp definately lives up to its Uber hype in the battle frontier. It has everything: speed, bulk, and power to really do damage to almost anything. There is not much else to explain about this as it is pretty obvious what it is supposed to do. I may consider switching this for a bulk Choice Scarf version for the Castle.

Wobbuffet @ SE reducing berry or nothing (NN: 13)
IVs: 31 / 0 / 31 / 28 / 31 / 0
EVs: 4 HP / 252 Def / 252 Sp Def
Nature: Sassy
Ability: Shadow Tag
~ Counter
~ Mirror Coat
~ Encore
~ Charm
This was the star of the show for me. 13 (nicknamed after the character from House, I love that show) can take an unboosted attack from pretty much anything and is always able to kill it with Counter or Mirror Coat. Wobbuffet was especially useful in the Battle Castle where I could check the movesets of the opponent and know which counter move to use. I don't think there were any battles that I needed to use Wobbuffet in where it did not get at least one kill. I highly recommend this pokemon, it did excellent for me.
Stragy/How I lost: There is no real complicated strategy for this team, just basically make smart decisions ._.. For the Castle, I made sure to check all the opponents pokemon and usually check their movesets and stay fully healed (in HP) before every battle. I lost basically because I ran out of CP and had to get stingy. This is mostly because of my stupid Rotom experiment which ended up being a huge dead-weight and costing me a lot of points. In the arcade, the only thing I would suggest is to aim for spots where there are 2-3 "neutral" conditions in a row rather than 1 good one- the roulette moves too fast to hit what you want to every time.
I have only made and used this team for one day so hopefully I will be able to make good changes and drastically improve upon the records I got today.