The cone won't appear until he's quite close, within 10 feet, and after he's moved a bit already. Can most fighters do 60 points with a single attack while inside an AMF? I doubt it.
What's the radius on AMF? 10 feet? It all depends on the cone's radius. If the fighter can start his action ten feet away from the cone, he could Leap Attack charge it and get +4:1 damage per attack penalty. With a high enough BAB, he could overcome that.
Otherwise, yeah, I think he'd be hard pressed, as multiple attacks have to overcome hardness separately.
Yeah, AMF has a 10 foot radius, and the "hat" I'm talking about is 8 feet wide. So he'll be 2 feet from it when it unshrinks. So unless he'd already initiated a charge, and the DM lets him finish it against the "hat" instead of the original target (the wizard), he won't be charging it.
I think I'm the one that came up with this originally, and it was for a gnome or halfling wizard, so a 4' dome that's 3 or 4 inches thick would be fine.
Edit: Here is the math. Since we don't know the density of adamantine, we'll use steel. The density of steel is 490 pounds per cubic foot, or 0.255 lbs/in
3. So a 4000 lb steel dome would take up 8.163 ft
3.
The volume of a sphere is 4/3 pi r
3. A hemisphere would be 2/3 pi r
3.
A hollowed out one would be found by subtracting the volume of the interior from the volume of the whole, so
4/3 pi r
1 3 - 2/3 pi r
23 = 8.163
Setting r
1 = 4, and r
2 = 4-x, and plugging in and solving for x, we ultimately get that x is almost exactly 1 inch (I guess I remembered my previous math incorrectly, or it was a bit off).
So our dome will only be 1 inch thick if we want it to weigh a pound when unshrunk. Even using standard, unenhanced adamantine, the fighter would need to deal over 40 points of damage to break through the dome, assuming he has an adamantine weapon in his hands when the dome unshrinks (or 60 points if not).
So yeah, it can't take too much punishment, but it does give you a brief moment for a Contingency to kick in, and maybe even for a Quickened spell, depending on if the fighter actually gets to attack it in the same round as he moves up to you with the AMF.
Or you could wear a thicker and heavier hat. For each lb of increased weight while shrunk, you add an inch of thickness while unshrunk, 4000 lbs of weight, and a bit over 8 cubic feet. You can shrink 2 cubic feet per CL, so a 9th level wizard could have a 2 lb hat that unshrinks into a 2 inch thick dome, etc.