I just want to start off by saying holy shit, I just finished possibly the 2 most frustrating demigod games in my life. Erebus maxing out minion upgrades, and sending them to attack towers, and even our citadel, which almost made us lose a game we completely dominated in kills, warscore, and upgrades. If there was more than 1 erebus on the other team using that "strategy", we would have lost. There is no viable counter, as the only way to prevent it is to kill Erebus, and he can sit in his base and send his minions to attack whatever, and they have so much HP that not even the best AOE in the game can take them out (Fire TB). There are 20 minions running around, all with 2000 plus HP, always being healed, that can't be taken out, and they can literally burn the citadel down in a minute... talk about cheese, and something that needs to be addressed immediately. It's game breaking, and I feel lucky to have won the 2 games I participated in where this strategy was used, they were extremely close calls.
If his nightcrawlers had 2000+ HP you were getting stomped anyways. Also, the maximum he could have had at one time is 16. A little less hyperbole, please.
Counters: tower attack, tower health, icy torch, mass charm, ground spikes, ooze beast + post mortem, pressure the minion erebus to prevent him from getting nightcrawlers, hammer slam, mines.
What needs to be nerfed is the unlimited range of the Horn of Battle. Its range should be no greater than that of an Oak's Spirit Ward effective radius.
Also, there is a bug with Horn of Battle and Ring of Might. After being activated even once, the effect automatically applies to all new summoned and spawned minions in perpetuity. It still expires after 10 seconds on each new minion. This is surely skewing the perceived power of the horn.