Well it would work like this for each battle your getting 12 EVs here-s the formula:

(y+4)*2. you didnt uderstand? well Y is the amount of EVs you get for beating a Pokemon in this case would be 2 PLUS 4 of the POWER ANKLET so far we have 6 EVs per battle BUT you have Pokerus so it would multiply by 2= 12 EVS per battle.

Okay .... so now that we know how much EVs per battle you get lets just divide (sorry for the math) the recommended amount of EVS for each stat would be 252 so lets divide now D:

252/12=21 confirmation: 12*21=252

RESULT: you would have to do 21 battles if my math isnt wrong...

NOTE: I DIDNT cont you 10 carbos