I would recommend Reuniclus as a good Psychic type to train. It's available easily in your game (on Route 9, but available as Solosis on Routes 5 & 16 and also in Strange House), has a very good ability (Magic Guard) and good movepool.
Item: Life Orb or Leftovers
Ability: Magic Guard
EVs: 252 HP, 252 Def, 4 Sp. Atk
Nature: Bold or Modest (is best)
- Calm Mind
- Psychic/ Psyshock
- Focus Blast
That's a competitive set, but since you've just beaten Marlon, you might as well take some time to EV train. If you're not interested in that, Reuniclus has pretty good HP already so it's able to take a few hits. Just go and fight Audinos, they're easy to defeat, give good exp. and 2 HP EVs per faint.
For your in game (before E4) set, I would recommend
- Shadow Ball
Since you can't get Calm Mind & Focus Blast until post E4.
Note that Solosis can have Magic Guard or Overcoat as its ability, so catch one with Magic Guard.