As ka10 said, you'll be relying on Zen a lot since at max level FC will leave you with 2 spheres, one short for another FC
Leave STR at 90, work on your VIT to reach 90-95 and maybe your INT.
If you can't kill your mobs with one FC I recommed you get something for non-interruptable cast time, like a Phen accesory or an Orleans Robe.
Once you get RD 10 your SP troubles will be virtually gone, also...on the way to unlock RD you'll get GT-Energy Gain, which at the prereq level for RD is enough to provide you with extra spheres for additional FC should you need them.
Keep in mind that FC is efficient when mobbing in a sort of high risk-high reward setting: the more mobs you group the more hits you will be dealing and as such you'll have higher chances of your RWH, HF cards and GT-EG to proc. (In some cases I have regained large amounts of HP/SP and most of my spheres after FCombo'ing a group of mobs)
And don't forget reductions as you have been told, use them according to element and race of your targets....Reduction gears are actually a must as you mostly tank in parties so is an investment rather than an expense.
Edited by Erydanos, 27 April 2015 - 03:08 PM.