In my opinion, starting fresh from level 1 is always easier if you want to EV train a Pokémon with a certain ability and nature you desire. Using EV reducing berries on a non EV trained level 100 Pokémon, should only be used if the Pokémon obtained has the perfect nature and ability, or accidentally raised a wrong stat, or is a shiny. Battling wild Pokémon also grants certain EV’s when fainted.
You can use the following to raise a Pokémon’s EV’s:
Fainting Pokémon
(Grants one EV per Pokémon fainted)
- Skwovet: HP
- Chewtle: attack
- Gastly: special attack
- Rolycoly: defense
- Gossifleur: special defense
- Rookidee: speed
Vitamins
(Provides 10 EV’s per vitamin used)
- HP Up: HP
- Protein: attack
- Calcium: special attack
- Iron: defense
- Zinc: special defense
- Carbos: speed
Wings
(Grants 1 EV per wing used)
- Health Wing: HP
- Muscle Wing: attack
- Genius Wing: special attack
- Resist Wing: defense
- Clever Wing: special defense
- Swift Wing: speed
EV training using jobs
(After reaching Hammerlocke, you’ll unlock a new set of jobs)
- A little while: 1 hour (4 EVs)
- Very short: 2 hours (8 EVs)
- Short: 3 hours (12 EVs)
- Long: 4 hours (16 EVs)
- Very long: 8 hours (32 EVs)
- Half day: 12 hours (48 EVs)
- Whole day: 24 hours (96 EVs)
Power Items
(Gives 8 extra EV’s when holding the item)
- Power Weight: HP
- Power Bracer: attack
- Power Lens: special attack
- Power Belt: defense
- Power Band: special defense
- Power Anklet: speed
You can use the following to lower EV’s by 10 points each:
EV Reducing Berries
- Pomeg Berry: lowers HP
- Kelpsy Berry: lowers attack
- Hondew Berry: lowers special attack
- Qualot Berry: lowers defense
- Grepa Berry: lowers special defense
- Tamato Berry: lowers speed
Source
I hope this helps!