Put it in the Poke Pelago Isle Evelup (LV 3) for 32 Def play sessions, and then 32 Sp. Def play sessions.`(use Pokebeans to speed this up)
Or
(Give it Power Power Belt)
SOS a Pokemon that gives 2 def Ev (Slowbro, for example), and stats increase like so:
10 first Pokemon, 20 other (First KO: 10, Sixth KO: 110)
Then
(take off Power Belt)
Then battle a new chain of Pokemon, stats increase like so: 2 first, 4 other (First KO: 2, Fifth KO: 18)
110+18=128
Hope I helped!
Edit: With Pokerus:
(Give it Power Belt)
SOS a Pokemon that gives 2 def Ev (Slowbro, for example), and stats increase like so:
20 first Pokemon, 40 other (First KO: 20, Third KO:100)
Then
(Take off Power Belt)
Then battle a new chain of Pokemon, stats increase like so: 4 first, 8 other (First KO: 4, Fourth KO: 28)
100+28=128
