Ok... I own this game and i know for a fact that you cannot beat him/her with only your starter Pokemon. you need variety. Brendon/May will always pick the better Pokemon at the start so...
If you got Treeko, she/he will get a torchick, so go get a Pokemon capable of launching water or ground attacks such as geodude or zigzagoon.
If you got Mudkip, she/he will get a treeko, so get a Pokemon able to launch fire, flying, ice or bug moves such as dustox, gyarados or torkal.
If you got Torchick, she/he will get a mudkip, so get a Pokemon capable of launching grass attacks such as nuzleaf.
On the first encounter
Pokemon Are Not Evil they just faint the opjectson of the game is to beat the champion in the Pokemon league
Yes.
latias
use psychic Pokemon and beat all the trainers to get more light
unova
Get Kyogre..
You have to beat flannery
fire types beat steal
i beat that gym with pokemon in their high 20s but i had a full team at that point, ideally you want your pokmon to be mid level 30s.
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