yes its not easy but its possible so just keep trying!
latias
Yes.
use psychic Pokemon and beat all the trainers to get more light
yes i fink it is possible
Get Kyogre..
You have to beat flannery
unova
fire types beat steal
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.
Get a strong water pokemon. I used Azumarill.
No
train all your Pokemon equally