How can I make a large section (around 100 blocks) into air, without having to create a command block to individually place //setblock air for each block. Thanks!
/fill command. The syntax is:
fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] [oldBlockHandling] [dataTag]
The sets of coordinates should specify the two opposite corners of a cuboid selection.
For example, to fill from x=0, y=10, z=0 to x=30, y=20, z=30 with air:
/fill 0 10 0 30 20 30 air
You can create a tower of dirt or any other material, and then keep on building from the 100th block on that tower. After that, erase the tower (erase the blocks beneath the 100th block). You can see how high it is by pressing F3 on Minecraft. It will let you know how high you are. It may seem like a lot of work, but that is the only other alternative I know of.
If you are into more fancy and intuitive editation of world, you can try WorldEdit mod. It has Forge version for singleplayer gameplay and also versions for servers.
The biggest advantage is quite easy selection of regions with clicking the border blocks. Then you can do quite a lot of operations with them, see the webpage documentation for more info. You don't need to know coordinates, it also works for set range around you with some commands.
With WorldEditCUI minimod you can actually see the involved areas you have selected.
However you need permissions (well, same or lower as you need to use the vanilla commands /commandblocks)