Still, there is a "hits boss" bit assigned to every spell, and for Weak, it is set (meaning the spell will not hit a boss) by default.
Assuming you are using
some version of FFIIUS, then the location of spell properties in ROM should be at 797A0 (ROM without header).
The bab-il documents detail the meaning of the data (
http://rb.thundaga.com/tob/spelldata.txt), except they list some bytes as "unknown." The byte you're interested in is byte 02. You'll notice that it's CB by default for Weak. That it is a value above 80 indicates that the "hits boss" bit is set, meaning the spell will
not hit a boss. To keep the same hit rate, but make Weak able to hit a boss, turn that uppermost bit off, or to put it simply, change the value of that byte to 4B.
(DISCLAIMER: I'm not 100% positive I've got the right byte, as it's been a long whole since I
didn't use FF4kster to make this change, so if changing that byte doesn't work, it's because I got it wrong, and you should try playing with other bytes in this table that the document lists as "unknown.")