I see so many guilds just outright defeating the other side, but you can earn so much more GP per battle. Here's how:
There are two basic positions in the battle
Vanguard (The characters you see on the screen)
Rear guard (Support players not shown on the screen)
You have one full hour to gain as many GP as you can. By destroying the other teams Vanguard so quickly, you make it so that you can no longer attack them which means you cannot earn GP. You also break the attack combo during the battle.
For the first 50 minutes of the battle, let the rearguard make attacks. They should attack one card at a time, and alternate between players. This will build GP and as long as the attacks are within 10 minutes of each other, increase the combo multiplier too. Try to work with your guild mates making these light attacks as often as possible to build up your GP and combo. The goal is to reach 100 with about 10 minutes remaining.
At 10 minutes, let the Vanguard (or Rearguard if the Vanguard has been defeated) attack with as much power as possible to try and defeat the other team completely.
Things to avoid:
Don't attack hard early. This just defeats the opponents Vanguard and makes it so you cannot earn more GP.
Don't attack too fast back to back. Remember you have 10 minutes in which a guild mate can increase your combo. Give them time to attack. You will be updated real time.
Don't worry about MP. If you run low, jump into a quest to try and build some back up as quickly as possible. Another trick is to save your lucky egg because when it hatches it will also give you some MP.