This is actually reminiscent of the whole homo vs DF idea and the problems in switching up gear. Do you react and screw up your build to be able to resist or do you make them screw up their build...

I'm increasingly convinced that for the really rich people, your best bet is to actually have ideally 2 different setups. A gear setup for casters and a gear setup for melee (you could be even more specific i suppose but...). With each setup you should be able to reach your breakpoints and be able to stack resists/absorb if needed. For instance vs a fireball I might switch out my DF to say a Spirit for the resists (losing a skill level but retaining FCR and gaining resists) or I might switch out my shako (because I dont need the DR) for a +2 fcr circ with life....that extra fcr might allow me to slip on dwarfs or a nokozan or wisps or whatever...I might neglect my White wand and use a HOTO for the bonus in FCR to again lose some fire power but be able to retain my casting rate while using things to absorb and gaining resists. If you're using the marrowwalk bug...well...thats your decison for damage so accept the weakness in your choice of boots vs losing an insane amount of damage. I'm still not convinced you really need to stack as much as Sov's V/t would simply because unlike Vangosroth you don't need to get in as close to still use your big guns (as opposed to his charge/smite since I'm sure people try to stack lightning res on him to nerf his foh)...you dont need to tank that many fireballs and if you do...well...you need some more practice in manuvering because its a lot harder to dodge some idiot spamming teeth in all directions than it is to dodge fireballs.

Basically all that rambling aside Im of the notion you want to have a gear setup in mind for casters as if you started from scratch rather than trying to patch things on...at least initially. This is why a good hybrid is increasingly dangerous to people if both their weapons differ in type and are still effective.