All I have to tell you, you should grind exp for a good while until all your pokes are at LEAST Lvl 30. The game is extremely hard if you kinda rush it or train only some pokes, so you have to train all of them equally and be careful with their movesets.
Almost forgot, be prepared for a quite tough battle, I don’t want to spoil anything, but be careful with everything. You must have noticed that Gym Leaders (Orion) do not go around 1 type, and that also means a wider moveset for their pokes, so be prepared for anything that comes in that battle and don’t think you’re safe only because your poke has a Super Effective move against theirs.
To Jeffmariaux. That’s a pretty nice idea since D. Munchlax/Snorlax has a pretty nice Atk stat. The problem is that Xavier pokes have some Physical moves, and D. Munch/Snor doesn’t have the greatest Def stat, and the biggest one is that it has very low Spd, and Xavier’s “special” poke has pretty high Spd AND Atk.
It IS a good idea, but only for a part of his team.
Unless you did that already and I’m just rambling around.