*sigh* well i could say my story is a general one for a new player of sorts...
i was playing ruby, got to groudon, and didn't freaking save before i faced him. yeah i know, what was i thinking. was wayyy too far from groudon on my save file to reset, so bleh i just let him die. later on, i had a great team that consisted of very high level pokemon, especially my beloved bubbles the swampert at like i think i got it to lv100, was planning on migrating them when i got there in diamond, then my nieces and nephew and their parents moved in, and i lost my ruby to them >_> havent been able to find it since. R.I.P. Bubbles.
(but on a side note, the firered game i gave my nephew for i forget what, they forgot and i took it back ;D)
since then, i have learned to keep a hawk's eye on my pokemon games, and ALWAYS save before you face a legendary.