Some questions then...
Does it increase the weaknesses of the following...
Darkness
Holy
Drain
Aerial
I don't recall any monsters being uber weak against arrows... or Darkness (there are a number of monsters, but I don't think it was 4x damage) or Holy (there's a ton of these) or Drain (none of these exist).
Also, can you tell me if the Golbez that Tellah fought is weak vs Holy and if so, 2x or 4x damage?
The only way Golbez takes so much damage is by being weak vs Holy @ 4x damage. I took a look at the spell power and compared to BSiron's (insanely accurate) FAQ for magic calculations.. it would take a character of 44 Wisdom to do approximately 9999 damage assuming that the target has 0 magic defense and is not weak vs Holy. Tellah has only 16 Wisdom.
I also want to know if Zemus was weak vs Holy. He probably isn't at all. Fusoya starts @ 40 Wisdom (remember that his starting equipment boosts his Wisdom, but 40 is the actual base).