If the teams truly are stacked, not a lot can be done except your best. As humans camp a bit more than normal, as aliens the same and either way, make sure there's a dedicated builder, because with a stack, the base is gunna get raped... repeatedly. Don't hope to win the game, unless by some miracle you manage to get S3 before they get S2, or you managed to get some good players join up and go into the slots left open by cowards specing out of aliens (this has happened to me on quite a few occasions).
Just remember it's a game, against a stacked team you'll most likely lose so don't play it to win. There's nothing riding on whether you win or lose, just play to have fun.