1. You can get another one at Wish Cave.
2. You don't. He/she gives it to you by the time you get to 99F of Purity Forest (you don't even have to battle Celebi, he/she will just join your team if you reach the last floor)
3. Bring one or two Link Box, two TM Frustrations (at about the 10th floor link them together, a lot of Max Elixirs and Reviver Seeds, and Gravelerocks (it has a 20HP set damage) Also you might want to try using Alakazam (SUPERB lvl.1 defense and sp. defense stats), OR Charizard (Heat Wave owns), OR Electrode (decent lvl.1 stats and learns sonicboom at lvl.1 which inflicts 55HP!!! SET damage, ALSO he learns it AGAIN at lvl. 15, you could link them together to deal a total of 110HP!! SET damage), Also try to beat as many pokemon as you can on the beginning floors so you could level-up and get stronger,
ONE more thing, I can ALWAYS rescue you when you get stuck in any lvl. 1 regression dungeon (no matter what floor you're stuck at)