I'm looking at this from a "worst case" scenario of a team full of Erubi. If it's a mixed team then you should be able to take one lane and fight over any leftover flags in which case they aren't out-warranking you early-game.
In this "worst case" the Erubi are pushing all your flags and you are hanging back at your towers mostly defending. They will get some early war rank on you, yes, but the idea is that you don't give up kills early game so you can make it to mid-game with a equal amounts of gold (maybe a little shy due to them controlling a gold flag depending on the map). Once in mid-game you should be able to start pushing back on Erebus in different ways depending on your demigod of choice.
Rook: Tower push him.
Torch: Harass with fireball (don't let him get bites off on you, just fireball and retreat).
Regulus: Harass with mines.
Unclean: Harass with venom.
Oak: Mid-game you should actually be able to push him toe-to-toe. If you get low shield. Alternatively, harass with penitance.
Queen: Same as oak, you should be able to take him head on mid-game, just play defensively, he should burn mana faster than you.
Sedna: Same, take him head on and play defensively (conserve mana for heals).
You may notice a pattern here. Harass him with ranged attacks. Erubus' strength lies in his bite (melee attack). If you just keep harassing him with ranged damage eventually he'll have to fall back. If he does take up bat swarming in on you then rest assured he is burning through mana like no other (mid-game) and he can't sustain it. Just keep running to your towers when he does this and when he turns hit him in the back with ranged.
The problem with Erebus late game is that he is really good at getting away. However, when you are fighting group vs group, one of them getting away means they left their teammates alone against you. So let him get away and run back to his crystal, you can stick around and kill his teammates (or chase them all off in the case of multiple Erubi).
Remember, bite is strong, but it's also a melee ability. He does have a bit of a speed advantage but on any map but Prison that shouldn't matter too much because you only need to make it to your towers.