Beat the elite 4 and get a good amount of pkmn. then catch kyogre(cave of origin), rayquaza (sky pillar at level 70), latias (above slateport and needs a trick to make it appear), and the 3 regis. (you need to solve a puzzle; the place to solve it is hard to locate.)
latias
Yes.
use psychic Pokemon and beat all the trainers to get more light
unova
You have to beat flannery
Get Kyogre..
fire types beat steal
train all your Pokemon equally
Get a strong water pokemon. I used Azumarill.
No
Swampert.
To defeat or "beat" the 7th gym leader in Pokemon sapphire there twins and they have a solrock and a lunatone to beat them I suggest about level 40-50 water types.