Prismatic Wall + Bigby's Forceful Hand.
Victims have to suffer all spell effects, should they get Bull Rushed. Have the Hand itself stop just in front of the Wall so it doesn't get annhilated (of course, this scheme can be tried by any Bull Rush).
Reverse Gravity + Prismatic Wall.
Similar idea. Cast Reverse Gravity, then cast Prismatic Wall on the ground- being weightless, it does not fly into the air. Cancel the Reverse Gravity. For added entertainment, have more than one wall.
Cloudkill + Forcecage (+ Dimensional Anchor)
I ruled that the hemispherical Wall of Force cannot be used to trap people: it is powerful enough as a defense spell! However, the trick can be played with Forcecage. Variants can be used with Acid Fog, Incendiary Cloud etc.
Teleportation Circle + Bigby's Forceful Hand.
The bottom of the ocean is a good destination. What's the water pressure down there?
Mind Fog + Chained (enchantment spell)
Since the Chained Spell will have an abysmal DC, then kick off with a Mind Fog.
Power Word: Stun (or Otto's Irresistable Dance) + Blade Barrier.
Mincemeat.
For the grand finale...
Chained Maze (using Rod of Greater Chaining [T&B]) + Dimensional Lock (T&B) + Blade Barrier (lots) + Forcecage.
When the victims return from the Maze, they are trapped with no recourse to get out. Instant death, no save, en masse, no hit point cap.