I thought you could only get A-support with one character per unit, tho.
Step 1: Don't get any supports at all during the main game, or at least until you have all characters.
Step 2: Save the game at this point on one file.
Step 3: Enter the first floor of the Tower of Valni, and wait for all C supports among the selected party to be formed. Then retreat
Step 4: Re-enter, same party, get B supports
Step 5: Re-enter, same party, get A supports
Step 6: Save the game to a DIFFERENT file than the original one. This tells the game's non-file-specific records that you got all of those A supports at some point.
Step 7+: Reload the original save, gather different A supports, and save to the secondary file each time.
Step Final: You now have 100% Support Library.
So long as you don't overwrite your original one, you'll unlock every support. This also works for FE7, so long as you save at the chapter where you get Renault (The chapter before Dragon's Gate), instead of in the final one... but it's much harder to succeed in those instances.
EDIT: Maybe it's time for a Fire Emblem topic instead of hijacking this one?