Alright good point. I've updated the OP. The thing with Volbeat, Illumise, etc. is that, since all MH users have terrible defenses, it's hard to switch them in, especially this generation, so I wouldn't get too overexcited about most of these guys. What makes Erufuun so appealing, in my opinion, is its Speed. Erufuun can not only Encore fast set-uppers with priority (like Infernape) on top of set-uppers without priority, but it can also switch in very reliably on other MH users (Volbeat, Illumise, e.g.) and shut them down completely.
Counters in my opinion would be:
Ice Shard Weaville is pretty much the bane of eruufuun and many Mischievous Heart pokemon like Volbeat and the others.
HG/SS pokemon with Magic Coat as they are unpredictably have access to it and can switch in and bounce back any phasing or status you send its way (bar Assist-Dragon Tail if it works). also a well timed DW-Espeon switch in, possibly if used correctly.
Extremespeed users such as Lucario and Arcanine can bother alot of Mischievous Heart users, unless they boost their DEF evs specifically.
For the Assist-Dagon Tail theorymon thats been going around, you can switch the mummy pokemon in on it after it uses the attack to change his ability.
those are some ideas
Herbivore Pokemon counter Erufuun pretty well.
And then what is Erufuun going to do? Switch out because it likely can't do anything to Herbivores past that unless it uses its last move slot for a non-STAB move.Not if they get Stun Spored/Encored/Taunted.
Not if they get Stun Spored/Encored/Taunted.
Conclusion: Espeon will be OU just to fuck with this thing.
Anything that can switch on an Encore, Taunt or something similar, has priority and can OHKO it or break its Substitute in one hit is a pretty good counter, switching a bit also works. Grass types to take the Leech Seed do well as well. Mirror Coat. There is no hard counter to it but there are many things you can do against it.
Imagine the turn order for that.Encore-->switch means Erufuun lives another day, though, so it can come in and try again once your Herbivore pokemon is dead.
Granted, the Herbivore Pokemon probably won't see much use in OU so Erufuun doesn't have to worry too much about them.
Deer used the same move that broke Sub on the same turn you spent Encoring it instead of switching out....Let me fix that.
Trainer switched in Deer.
Erufuun used Substitute
Deer used a move.
Sub fades.
Erufuun used Substitute.
Deer used move.
Sub fades.
Erfuun used Encore.
Yeah.
Deer used the same move that broke Sub on the same turn you spent Encoring it instead of switching out.
SUPER EFFECTIVE!
Uh oh, I don't think so.
Then let's look at the situation as if it wasn't a revenge switch in.
Trainer sends in deer.
Erufuun subs.
Erufuun used ??? <-- What is it supposed to use? Stun Spore = activate Herbivore, Encore will fail, Taunt probably would stop it from buffing, not like it would need to to trump Erufuun.
Toxic!
Deer used Megahorn.
Sub fades.
Erufuun used ENCORE? WHUT?
Deer STILL uses Megahorn.
Erufuun fainted.
I'm not seeing how you could Encore AND switch out unless you had Escape Button in the case you survive a hit.You'd pretty much just be stalling, but if you lock your opponent into Megahorn, that's a free switch for you.