Hmmm....
Make a layer-cake.
Dig out a big chunk of rock until you get a large cube of exposed stone and flat ground around it.
Now dice up the cube. In the middle dig out a big chunk the same size as the middle section you decide to use but don't remove the surrounds, just up-ramp down into the cube in a big square, build bridges for seals, use a linked support/wall to keep the center chunk up and remove everything under it in a block the same size but 1 z shorter.
Fill that with water.
Now go out, repeat the process and leave a solid wall around the center layer, seal that down at the level of the central water cistern, and then dig down far enough that you can sink that wall down to ground level, attack a support+floor to hold it up and remove the bottom layers, fill that with magma.
Now the last level, attack bridge seals around the outside edges, fill the space between it and the inner hollowed out piston with more magma, attach another floor+support to hold up the outer layer, then remove the bottom and, yep, fill it with magma.
It is up to you to determine how to get stuff into an area around the cube which is closer than the walls are high, but once you do this, drop the walls in sequence, outer, inner, this will flood the area with magma pretty rapidly but it won't be A LOT of magma, it should work out to be just a bit more than it should take to fill one layer to 7/7 or so before it spreads out.
Now that everything has taken a magma bath, drop the central block, teleporting a big column of water up where it was, and this is important: placing it one level higher than the magma around it.
With luck it should splash down and flash everything into obsidian.
Are there easier ways? Naturally, but this just seems more fun to me for some reason.
Outside
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