For STR-based weapons (everything except bows), every 5 DEX increases ATK by 1, while every 1 STR increases ATK by 1.
For DEX-based weapons (bows), every 1 DEX increases ATK by 1, while every 5 STR increases ATK by 1.
Dragon Breath does not take into account your stats for damage. It takes into account your current HP, max SP, Dragon Breath skill level, your base level, Dragon Training skill level, ranged damage modifier, and elemental modifier.
DEX will help with your variable casting time. If you're wearing Temporal DEX Boots and have 120 base DEX, you will also get the effects of the Temp Dex Boots (reduce fixed casting time by 0.5 seconds, increased ranged damage by 5%).
No difference in damage between Main Gauche and Mace. Mace is unbreakable by default, though.
Main Gauche also has slightly faster ASPD, which should help with reducing animation delay by a little. Usually not noticeable.
"Best" is subjective. Maximizing your HP/SP is important, which will increase your damage per cast. However, reducing fixed cast time (through Temp Dex Boots and 120 base DEX) and reducing variable cast time are also important, as they will increase your DPS (cast faster). Depending on how far along in the game you are, you can also focus on reducing skill delay through +9 Tae Goo Lyeon, Kiel-carded headgear, and Expert Rings. This will also increase DPS, as you wait less time between casts.
Edited by MorseCoder, 13 February 2015 - 06:28 PM.