Back at 70th, we could trivialize this heroic boss by having everyone move as a group.

The elementals randomly pick a person and follow them for a time. If everyone is all over the place, then a straight line to any given character creates a fire trail which then sucks to avoid.

Our solution was to bunch up. We used a raid icon so everyone knew who to follow. You'll probably not need that, since you're playing everyone.

We made really huge clockwise circles around the room. The direction doesn't matter as long as everyone is going the same way.

No matter which toon a given elemental is following, they're all basically in the same place and moving the same way. That makes it trivial to follow them. The boss moves at the groups speed, while the elementals move much slower.

Prior to finding this, just about every group I had run with in Heroic Mech just skipped the fire lady boss and did Parthenon. Aside from the badge, she doesn't drop anything extra on heroic which you cannot also get on normal. You don't even have to pull her on normal or heroic.