https://docs.google.com/spreadsheets/d/1YzSX2XuH0MzfYGVlQHbW304UunGx9xd_HP0ZriuG1cY/edit?usp=sharing

The spreadsheet here is quite massive but gives details on which weapons and spells are better suited for which enemies. This should help newcomers to the run figure out which gear they should use.

The spreadsheet is basically another way to look at information from https://www.gamefaqs.com/genesis/563334-phantasy-star-iv/faqs/23221

Basically every weapon and spell has an "element" (for lack of a better term). There are 14 elements in the game and each monster has a strength or weakness to each of them. A "0" means they are immune to that element, a 1 means they take 1/2 damage, a 2 means they take normal damage, a 3 is 150% damage and a 4 is double damage.

There are 3 weapons that have the F6 element (the "Light" Element): The Elsydeon, the Silver Tusk, and the Psycho Wand. ALL enemies in this game will take full or increased damage to this element, which is why those weapons are so good. There are also 4 techniques and 1 skill which share the element, all of which are used by Chaz: Tsu, GiThu, NaThu, Rayblade, and Megid. Keep in mind that the Profound Darkness' Megid is the same ability, and there is no item in the game that can mitigate that damage because of that.

Debuffs and Instant-Deaths are also considered in their own class of "elements", usually F8-F14 (minus F13 which is categorized as the Holy Element for St. Fire and Efess). Enemies with a 0 in that column indicate no effect by it, 1 means a low-chance of success, 2 means moderate chance, 3 means almost guaranteed. Each boss, which the exception of Iglanova, has full immunity to stuns, debuffs, and instant-deaths (unfortunately).

Use this chart as you will. Hopefully it provides some insight on what to use. Calculations are done with sort of an average number, but regardless of the number used for calculation, you should be able to see how each weapon and ability fares against each enemy.