even without any mag increase buff, fire hits better magical and physical damage than light, and also has longer ranged nukes, 3 nukes instead of 1, magical defense, fire wall for vs 2hs, magical defense, immunity to ice, etc. And lightning is...weaker and gives speed For more in depth, click below lol.
Fire/Ice/Weapon int is, despite what many think, BETTER than light/ice/weapon, for many reasons.
1) The imbue (which is where magical damage in physical attacks comes from) is 13.18~26.00 % stronger than the lightning imbue at 100. Basically, this means that, on average, fire, without any "piercing force" buff at all, hits 19.64% stronger than lightning imbue. And lightning buff only increases damage by 18%, so you hit as if you had a buff 2% stronger than force piercing level 3 without even having a buff :P.
2) Fire gets the skill "Fire shield - Emperor", which means all you need is a total of 22% immunity of a status on your accessories and you are 100% immune. Without fire, you can't be immune.
3) Fire gives you a magic defense buff...need I say more here?
4) Fire nukes are stronger, longer range, and more plentiful (there's 3 instead of 1) than lightning nukes.
5) Most people disregard the physical attack buff because you are an int, but even as a pure int, assuming you are full blue, 35% of your attack comes from physical. So a 19% increase in physical damage still increases your weapon skills by 7% damage. And that piles on top of the 19.64% more from the imbue, so basically, your weapon skills hit 126.64% base, whereas lightning hits 118% base.
In conclusion for damage, this means fire hits 7.32% more damage than lightning does with weapon skills. Although the phys buff does not effect nukes, the nukes to begin with are stronger, and, averaging hellfire and disitegrate (which average 7.5% stronger than crane's thunderbolt) your nukes will hit 7.75% stronger because of cloud fire force.
So basically, it's stronger, has better defense, can operate from longer range, and is immune. The only thing lightning has better than fire is the 70 or so parry ratio you get, but 70 parry ratio doesn't make nearly a big enough difference to matter (for tanking, vs int mag def buff is better, and vs str fire wall helps tanking more)...and speed. But if you can learn to live without speed, I'd go with this build.