Ya. it will take awile to do it but ya. you should use some TM also.
Stark Mountain
Ground.
Train your pokemon.
Repels make WEAK not strong Pokemon not appear
yes.go to fuego ironworks and train in the grass
Cheat.
yes.Put the weak Pokemon first in the party.Then do a battle switch the weak Pokemon with a strong Pokemon kill the wild Pokemon the weak one will get credit.If you give it an exp.share and do the same thing the weak one will get even more credit.
The weakest Pokemon in diamond is a magikarp. It is also the weakest of all the Pokemon existed. Magikarp is weak but it will evolve to a mighty gyarados.
probably turtwig. grass Pokemon are weak full stop.
capture the legendary and train your Pokemon
Once you have the Exp. Share, give it to a weak Pokemon and have A strong Pokemon and two weak ones. Put the weak one witout the EXP. Share in the front and go battle. Switch out the weak Pokemon and then battle with the strong one. Then both weak Pokemon will have exp points.
There is no best Pokemon.All Pokemon are weak and strong against others.