I can say that Erebus has one of the longest warm-ups in the game for his Mass Charm. I think the only slower effects are the Tower of Power and Hammer Smash. I believe, it's just over 2 secs.
Learn interrupts. Certain attacks will trump others. Knowing which attacks can "block" another is important. When an attack is interrupted, I believe the mana is spent, the ability is activated and must cool down.
As Sedna, start off with Silence or Pounce (though Pounce will be VERY close). You might get both Erebus and UB and be able to run. With Sedna, you have an inherent speed boost which you'll need.
As Oak, use Penitence. Once the circle of light starts, Erebus can't cancel, You'll interrupt him and he will be open and Vulnerable. Again, the the mana is spent and ability is still invoked and must cool down. Then use Shield to avoid the UB. Or take the UB's hit and Shield, pop a potion, and Work toward Friendly support.
Frost TB can use Frost Nova, then Rain of Ice, Finishing with a 100 yd dash.
As Erebus, use Mist to avoid the initial attack, Bat out, or prep to drop a poisoned pot.
QOT has bramble shield to block most of the damage. Fire TB, Rook, and Regulus should run at the first sight of UB or Erebus.
Depending on the map, you might find their tight integration to work to your advantage. On a larger map, they can only be in one place.