I just realized that Cade in Gargantuan mode bypasses the majority of the problems we would otherwise have, including visibility.
Rule 11: everything is air-droppable at least once.
Here's how it works: Treebeard is 300 feet up in the air. Given that on the first round Cade would drop 150 feet, and on the next 300, my plan is to DD him up 120 feet above the guy, move up to RJ while he grapples in freefall, then on the next round, use Tactical Teleportation to bring people to ground level, whereupon X could extract us.
If I had used 3rd-level slots to fuel that Evard's, I could use Tactical Teleportation then regular Teleport instead, but now the damage is done.
EDIT: Notes on why he bypasses most problems:
At Gargantuan size, there's a good chance the vicious winds won't even affect him. Most spells that deal with massive winds spinning about have rules dealing with creature size (i.e., "at this size creatures are unaffected").
He'll also be too big for the tentacles to grapple. (Gargantuan is EXACTLY two size categories above Large.)
He stands the best chance of grappling the big dude. Even taking into account the darkness, he has the highest hit ratio here.
As long as I make sure he doesn't drop all the way (which would make a satisfying 'splat' sound as far as our enemy is concerned, but it could do so for Cade as well) he should be in the clear. Since Phaedrus ruled that kinetic energy isn't conserved after teleportation, Tactical Teleportation provides us with a nice air-dropping recovery mechanism.
In the words of Agita's sig, it's all about reality and making the world conform to that reality...
...by dropping a fucking house on it.