The event ends when one flag (top flag) gets to 0%. The Goal is to defend for as long as you can. Use your attack and health boosts. At first you will send your full fleet to a single flag and then you will quickly split your fleet into 5 equal (balance zombies and titan) fleets which you can send to 5 different flags. Pick the top 5 flags on this list. When your battle at the flag is over you need to withdraw your troops immediately and send them to a new flag. Keep picking the top flag on the list, which has the least durability. Towards the very end of the event if one flag is less than the others you might join your fleets back together to defend the lower flags. Make sure all of your troops are in battle getting points!!
Sometimes its better to not send to the top fleet if they are already in battle and can win the fight.. If your fleet time is 15 seconds, it can be better to send to a different flag with less people.
As you get further along and the alliance is more powerful, this event will take up to 45 minutes. Its the worst event. When you get more powerful (750m to 1B power) you will beat all 15 levels. You can start skipping levels to shorten the event. If you skip most of them, you can finish in about 20 minutes.