I have no interest in showing you how for the following reasons:
1. It is a thought experiment, don't take it so seriously.
2. This would never enter into an actual D&D game so don't take it so seriously.
3. Nothing, but still, don't take it so seriously!
Whoa whoa WHOA! The INSTANT you start to say "Don't take x seriously" in a thread where intelligent individuals are obviously seriously discussing something, is the instant where you should STOP POSTING.
Edit: @ Rakusia: This is being done, not only as plot, but for a power so unquestionable and unfailing, that even gods look at you with worry at what you will become... oh




, would building this thing make enemies of gods?
@ Deathstar: This is a very valid point. Now the project has gone from making reflectors in close orbit to doing such as well as making a focus lens closer to earth. At this point, it would all have to be connected together, but this point actually makes things a lot easier, as you would only need to focus each reflector onto the attached lens (no changing calculations). At that point, the only problem would be keeping it in orbit while being close enough to the planet not to have your "death beam" turned into a "slightly annoying heat beam" by solar winds/radiation/etc.
I think now that we are approaching the fourth page, that the OP needs to be edited to include what we have discussed.
First, the moon sized mass is not necessary, only the mirrors, focus lens, and connecting material (a latticework of titanium bars would do nicely).
Second, since the lens would focus all the gathered light energy into a straight beam, all you would need to do is move the construct above where you wanted to hit, then angle the entire thing as needed.
Third, we will assume, through shrinking and growing, that getting to the necessary speed will be plausible through the use of a single high level spell designed for the purpose of accelerating a small mass to ridiculous speeds.
Fourth, since the whole think can be made in outer orbit, pointing away from the earth, that no arch-mage will spot it and have his curiosity ruin our day.
The one thing that needs to be considered is moving the thing while in orbit. Simple nudges of gravity would do the job for rotation, but to get it in the right spot to hit where you want, I think a necessary skill check would have to be made along with the spell-casting. The spell casting is just the "hands molding the clay". There is still room for human error/unpredictable variables. There needs to be a critical fail chance.
I think the plan seems very sound. The original idea was a good base, but I think that this is the realistic conclusion. Also, a custom high powered spell for altering levels of light could add power to the thing by increasing the light at the focus point tenfold, theoretically meaning that the device would only have to be one tenth the size. I think keeping it from getting too big is a goal that should be considered.
With all these things considered, now we just have to choose the spells we need, and mention what ones would have to be made more powerful and mention in what order they would have to be done. Again, that is something that should go on the first page, before this thread gets big enough that people only read the first post before commenting.