From what I can tell, if you have a single item with max resist to a status, then you have max resist to that status. I'm not entirely sure exactly what "max resist to a status" does, though - I *think* it just reduces the time that a status effect lasts to some minimum value, but does *not* give you any damage reduction for things like the Fire DoT.

As for shield defense - that value only affects how long it takes your shield to break when hit with that type of attack. So if you have an Owlite series shield with Normal/Elemental defense, your shield would be able to take more Elemental hits than Piercing hits before failing. If you're facing monsters that do Elemental, you might want both Elemental on your armor to reduce damage when unshielded *and* Elemental on your shield to take more hits before the shield breaks.

Note that some people prefer shields like the Stone Tortoise/Omega Shell because while they only have defense against Normal, they also have a lot of health to soak up hits of other types.

(Side note: The *best* way to avoid damage is to not be there when it happens. Having good defense isn't enough in the deeper tiers - you *have* to learn to dodge enemy attacks as well, since regardless of how good your armor is you'll still be taking some damage, and the game never seems to never drop hearts or health capsules when you really, really need them)