So as you mentioned your Pokemon doesn't have pokerus and is at the front of the party ev horde battling. It would gain whilst holding the power item; this for example:-
5x Weepinbell with Power item after being KO'd = 60 points a time.
Here's HOW it works:-
Weepinbell = 2 attack ev multiply by (5x because you're facing 5x weepinbell) = 2 x 5= 10 ev's earned.
Power items add 4+ ev's to a Pokemons values. For example:-\
Weepinbell = 2ev's per Pokemon - 2 x 5 = 10 but now power items add 4+ to the 2 = 2 x 4 = 6 x 10 = 60.
If your a little confused read over and over until it begins to make sense..... It's a little conufisng at first glance but helps later on.
For a list of horde battling hotspots I use this site HERE if you find it useful, add it to your fav's list on computer.
Hope I Helped!