AoE is the best way to go, or fight him at the towers.

TB: Like, everything but Fireball and Deep Freeze 
UB: Ooze, Putrid Flow
Regulus: Mines
Rook: Towers and Hammer Slam

Oak: minions and Surge of Faith?
QoT: ground spikes, spike wave, minions
Sedna: minions?
Erebus: I'm guessing mass charm and your minions... haven't played much with him

im asking specifically about oak...surge vs 2k minions is laughable.  is parasite egg viable? ya erubus should be easy to gank in this situation, but no one can stay in his minion swarm long enougn without getting raped to dent him. 

Surge will put a nice dent in his minion's hp, but more importantly it'll buff everything on your side, too. If you make sure to fight Erebus where creeps clash (obviously he's not gonna run into a wave on his own), you can use Surge to wipe half the creeps on his side, and buff yourself, your minions, and your creep wave.

An Erebus putting so many points into his minions probably won't be as strong as usual... If he's maxed out both lines of his minions, mass charm to delay yours, Bat Swarm to teleport out... you probably have enough points to either max morale and cut his minions to shreds between Charms, or put points into Shield to just wait out the Charm. If the Erebus didn't max both minion lines and Charm, your minions are probably stronger than his anyway.

make sure you've bought at least Building Damage 1 upgrade from the citadel to give your towers AoE. then run his minions into the towers and let them get zapped. if he's not microing them carefully enough they'll just follow you in, even if Erebus himself doesnt.

 also, buy the Idols to help fight. minotaurs will tank the nightwalkers for a little bit or at least body block them. the pathfinding AI is poor enough that having some guards stand in front of you will buy you like 5 extra seconds. priest idols of your own are a giant help as well. minion damage can be high but it is sustained damage not burst damage, you can get armor items and healing effects and withstand it most of the time. nightwalkers also only ever move at speed 6.0. you can personally kite them a bit with Boots of Speed. minions are poor at attacking while moving so just staying mobile will greatly reduce their damage against you. 

I think people might be starting to realize the power of minion builds.

-Drain its support. My Sedna build, for example, gets the +24 hp/s aura, which also buffs priests. By late game, my already-poewrful minion army is being healed every 8 seconds by a ridiculous amount by the two priests. Even on other generals, priests are amazing support. Take out any priests, they will make life hell for you. If you have a Sedna on your team, tell him to get counterhealing ASAP. MAKE SURE TO TELL HIM IN CAPS LOCK SO HE KNOWS YOU'RE SERIOUS.

-Kill the general, fast. A duelist Oak is terrifying to a minion build because he can drop shield to keep himself alive in the few seconds it takes him to burst damage me from full to nothing. Anything that focuses on doing a lot of burst DPS to the general directly will work.

-AOE. There's a lot of cool stuff you can do; parasitic egg and magnus rod are two examples of things everyone can do. It wont kill the army, but it will weaken it significantly. Make sure you kill any healers before doing this, or your attempt will be fruitless.

-Depending on the army, you may just want to attack it. It's possible that the general focused on speed/health gear, neglecting +mana. Focus fire on his army...after the priests, take out the general's unique summons (like shamblers, yetis, etc) as they generally cost the most mana to replace. This wont work if he's stacked with mana gear, but many times they neglect to do this.

EH?! What is it?!

Parasite Egg

ya my issue with the egg is that if youre doing that much dmg to the general hes most likely dead long before you will killthe minions.  but ive nver used it so

you're absolutely right Orion.

on that note i find that Cloak of Elvenkind is amazingly good at beating minion armies. it gives both evasion and armor so you'll take alot less damage. it also gives +20% move speed and short range warping so it becomes very easy to avoid minions and to catch their generals. 

Hit them with hammer until they cry.

 P.S. The Rook gains good AOE against small targets with God Strength.

Even though he can cleanse a parasite egg easily I think you could still get some mileage out of it with certain heroes.  For example an ice TB could hit it and immediately RoI, which is going to be 1800 damage up front to them, an auto-attack can finish them off if he's not super quick on the mist, and if he is you're forcing him to use an emergency tool at the start of the fight.  A rook with bould>hammer could probably put some hurt on him too.

That being said, the minions are super annoying and you've got your work cut out for you unless your team is really aoe heavy.