I ll try
The first thing I did was look for the celebi and think how he could be effective in this metagame.
It really is resistant, and you can enter in much of the metagame.
But you need to use leech seed very well to he be good in a match, it really helps to press the opponent, but has few pps.
So I decided not to use celebi, but to prepare something strong against him and snorlax.
I used this team against Conflict in R1 and Isa in R2:
http://p.shrib.com/b1pobZim?v=nc
A basic GSC offensive team, but I thought quite about the movesets. I could use drill peck in zapdos, but I'd be weak to rhydon. I realized that celebi could enter in Cloyster in sometime, so I thought ice beam would more useful than toxic. Egg usually attract two types of Pokémon, A steel-mon (Forretress or Skarmory), Celebi and Tyranitar or Umbreon. I wanted to somehow weaken or press Forrestress and Skarmory, so I tried using HP Fire, but I'd be so weak to Tyranitar and Umbreon. In this case sleep powder would help a lot. I love drumlax <3, and I think he fits very well in this team. The biggest challenge was deciding between Lovely Kiss or Earthquake, but I wanted to try to use Earthquake. This Steelix is standard, and the Gengar also.
I tried to summarize all
Here is the other team:
http://p.shrib.com/LfwAT6Ij?v=nc
I used against conflict in R2
I tried to make a cteam here, and it worked uhu
This team is weak to marowak, but I had to bet on it.
Forgive me if I did not explained right, but my english is bad and i'm a new player in GSC (I started learning in this tournament).