You want 105fcr
Griff 25
Viper 30
Arach 20
Magefist 20
Amy 10
= 105
The extra fcr on insight wouldn’t help reach the next fcr bp which is at 200

On the other hand, here’s the massive advantage:
Infinity on you = extra 45-55 light pen
Insight on merc = tripple heal aura combined with prayer and cure
You’ll also want to get 95% ES, 20 in telek, and dump all your stats in energy. At this point, you don’t even need res anymore. You just want a big mana pool, as 95% of the dmg gets redirected to it, and the 5% left taken by your hp pool is mostly negated by the mdr on viper, while any dmg taken is quickly healed by your merc’s tripple heal auras.
Poison is negated by Cure.
The only dangers left are:
Stuff like souls that can quickly burn all your mana.
The blood mana curse casted by succubus that cause you to lose life per mana spent, so you just need to avoid casting while it gets removed by Cure.
Against most things you’ll feel invincible (how much depends on the quality of your gear), but there’s always a limit to it… you can definitely die if you play poorly.