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.
Nerf it, reduce the max number of minions erebus can have, that might be the best solution.