Didn't have internet while typing this, so I can't search the thread. It's annoying.
Okay, let's look over what we can actually buy and how it could help us here.
Lets start by ruling out the simple things. We can't afford anything in the shop, and flight is already at a milestone and high. Thermal Control is not very applicable.
Infinity Edge: we could level our weapon as high as 25. The damage and MM aren't that significant (700 and 250 more, respectively), but they do help, and more APT is always good. Also has the possibility of an ability, I guess. A decent offensive option that also helps with skill negates.
Plasma Sphere: at 1500 damage per level, this could be scaled up till it rivals Juggernaut Wave in raw damage. Max level is 18. I don't expect another ability, with the possible exception of collateral damage. Not of much interest here as it remains behind Juggernaut Wave and a Magic Surged attack in raw damage, but might be better if we get an intense heat modifier. Melt could maybe help if it has some ridiculous amount of HP.
Thermal Spears: scales at 500/level, with double cast. If we get spell boost to trigger, they do quite a lot, but otherwise do less than Plasma Sphere. They don't seem particularly advantaged against this opponent, either. Trying to freeze it could help, but i don't think levels would help much.
Halt: essentially a gamble on something new at level 20. Halt is incredibly useful here, in theory. Though A&O would remove it, it still should let us immobilize the thing, Magic Surge hit it, and repeat. Don't even need to spend EXP to try that.
Juggernaut Wave: The raw spell damage option. Also helps with CC prereq, I guess. Possibly abuse knockback to keep it at range? A good backup plan if halt doesn't work. Also has control issues if we level it, though,
Defenses: The current plan. I admit I suspect this thing will 1-hit us anyway, but they can't hurt.
The current plan I'm assembling is to try Halt-Magic Surge hit-repeat. Probably with our aura on for extra damage. If that fails, either blast it with Juggernaut Wave to keep it out of melee or engage it. Possibly try the freeze thing somewhere in there.
I'm leaning toward leveling our weapon. Extra APT help a lot for both damage and defense, via magic surge and skill. That said, other things should work.
From what I can tell, we are both being really aggressive/competitive about this, so I'm tentative about the more defensive knockback plan.
So...
[x] Plan Guillotine
-[x] Halt it, then attack it with magic surge. (Try to make sure it's a two-handed blow.) Repeat as necessary.
-[x] If that strategy can't work, play keepaway with juggernaut wave and fire projectiles while you heat up the area, then drop Plasma spheres on it.
-[x] Have ignition Aura on unless it harms Sabina.
-[x] Use A&O if there's a spell.
Can't remember if it hurts her or not and can't go check atm.
[x] EXP Plan The Best Defense
-[x] Infinity Edge 20 -> 25 (1100 exp)
Honestly the defenses exp plan should be fine too, as should leveling a spell. Can't really know which would help the most here.