You will need these things:

• Atleast 7 Seed slots
  
• Repeater
  
• Tall-Nut
  
• Spikeweed
  
• Magnet-Shroom
  
• Coffee Bean
  
• Torchwood
  
• Lily Pad
  
• And of course to have unlocked the Last Stand Mini-Game

When you've got it all you start the Last Stand Mini-Game.
You can't get sunflowers or Sun-Shrooms in this level. But since you start out with 5000 sun to buy plants for and get 250 sun every time you get a flag, it's not a problem. There is 5 flags to get. When you get the 5th one, you win.
You choose the seeds that is on the list and you place the plants as shown on the 1st screenshot.
Then you just keep replacing plants if they get eaten.



And if you want to, you can start placing extra Repeaters and Spikeweed as shown in the 2nd screenshot.



Now you just wait until you win.

Good luck ;)

P.S. There is probably many different ways to complete this Mini-Game. But this was the one that worked for me :)