gardevoir, altaria, delcatty, magneton, and Roselia
Wally does not become the champion, but he is in victory road and is pretty powerful
You can battle Wally a second time after defeating the Pokemon League.
after each victory you get against the pkmn league
once you have helped him catch a ralts he will be at the end of victory road
At least level 56-65.
Wally will occasionally be at the end of victory rode for rematch
After you beat the elite 4 you can battle wally at the end of victory road after you beat him exit then enter and his Pokemon will be stronger
It depends on the levels of your pokemon, and the ones Wally has. This is in Victory Road, so I assume he has level 40-50 pokemon. If yours are around this nature also, you shouldn't have a problem. But, seeing as you said Marshtomp, instead on Swampert (the final evelution), I'm worried your pokemon are much to low to even be at Victory Road. Anyway, the NPC's of pokemon are always using terrible moves etc, so it should be a breeze.
Wally goes to the Victory Road before you challenge the Elite Four and stays there.
The victory road after you beat the elite four
To train at different places to beat you. He suddenly stops growing his Pokemon and stays on victory road after he comes back to the same spot. He's then just a quicky extra for experience points every now and then.
By challenging any trainers and pokemon that come your way. If you challenge them it will make your pokemon stronger. It is all a maze actually and you just have to walk around until you find the exit. You will know it is the exit when Wally comes to challenge you. Beat him then you can come out of victory road and challenge the elite 4.