Frankly any AOE ability in the game can counter Erubus minions without too much trouble with one exception. Erubs uses horn of battle and his minions become invincible; at that point there is no hero in the game that can stand toe to toe with the minions and you are boned (unless you get your teammates to hammer them with another AOE at the exact same time). Yes it's OP but instead of trying to complain here are three counters to this this I have found to be effective.
1. Ignore the minions and target Erubus himself. If he has gone full minion build Erubus himself will be pretty weak. Try to kite minions and hit him while keeping distance from minions. If he has bite eirly on his minions are not going to pose the same problem and can be smashed by most heros before targeting him.
2. Yes using horn make minions invincible but in between they are venerable and horn is on a 45 sec cool down. Wait until horn run out then smash the minions. He will constantly have to replenish his minions and because he is using a pure minion build is Erubus is very week without them. Smash him.
3. Beef up your towers damage to level 2. Level 2 health is not a bad idea either. Not the cheapest of counters but, it can certainly put the hurt on his minions. Depending on how he has built his minions, level 2 towers with a nicely timed AOE spell can sometimes kill his minions even with horn activated. Still it's expensive but much cheaper than Artifact counters which is ultimately what you need but are usually too impoverished to get, since you are pushed around a lot.
These are really your only options for dealing with a Erubus pure minion build and even then its really hard to deal with. Ultimately though if you have the gold the only REAL counter to this are Artifacts. Girdle of the Giants is the best counter (makes all auto attacks AOE) and Ashkandor rocks for someone like Regulus, but honestly. If you had the money for those items in a regular game we wouldn't be having this discussion.
P.S. On a side not I have had someone mention to me that Regulus using Angelic Furry and using Girdle of the Giants doubles his attack power (or AOE attacks at least). Can anyone confirm or deny this?
Remember if all else fails ganks work wonders 