The battlefield is split into the lower and upper halves, as you are aware. Summon your creature (you have made a creature by now, yes?) to keep her amused for a bit. She doesn't do major damage while she's invincible. Find the bridge section and go out on it a bit.
Snoop around by the edge and you will see that you can drop off the edge to a lower ledge of rock underneath you. Do so. From that ledge, look around the edges of it and you will find a place where you can drop yet lower.
On that ledge, follow it around to a third transporter that you can activate. That will activate all three transporters. You can take it back up to the top. From there, the two upper transporters will teleport you back and forth from one to the other.
You stand near one transporter and attack her. when she teleports, run back to the transporter and warp to the other one to attack her again. My character is a caster, so I just stand right next to the teleport while attacking and rapidly switch from one to the other. Rangers can do the same. I'm not sure how a warrior would do this, but the same idea holds. Since you can now instantly move from the upper to the lower tier as well, she can't heal fast enough to stay ahead of the damage you're doing.
At this point, she will start fighting back in earnest, but you can now defeat her.
