After you have defeated Eneru on the first island in Part 1 (Treasure Among The Waves) and returned to the ship, Franky will tell you he has made the keys to Choppers' lab and the kitchen. From there, find the pickaxe located on the upper floor of Sunny (the ship). After you have the pickaxe, go back to the first island. Turn to the left to find two palm trees. Use the pickaxe around the middle of the trees, and a chest will appear. When you open it, you will not see any text. After the light turns normal, you can go back to Sunny, save the game, and exit to the title menu. Go to Survival mode, and it will now have two options: "200 Down" (kill 200 enemies to win) and "Boss Rush" (kill 50 Bosses to win).
Start with Bartholomew Kuma to win more easily.