If the reason was that one character was dominated, I suggest the following.
In this race, you need only one character to reach the doors. So I always detach one and let the others wait outside the big gate, out of reach of nautiloid cannon. Equip the PC that does the dash with sufficient misty step and invisibility spells (potion, scrolls) to reach the door without too much of a fight. Once past the door, the battle is done and the rest of the party can move in to join the champion (and heal. Don't let the champion heal before the others have arrived)
