A few exploits brought to my attention.

Exploit 1:
It seems it's possible to bypass Weakness on WHM, SCH, and SMN by casting on the targetted person as they're raising (Getting up). In some examples, I've seen Medica II used. I'm sure it's just casting in general.

Exploit 2:
In the quest, "In a Titan Spot," it seems you are able to bypass damage completely by being current target on Enmity List and being entombed in a gaol. I'm not sure if this is valid in The Navel (Story), best to investigate both instances. I'm not sure if this is working as intended, but it seems like it shouldn't be there.

Exploit 3:
In Amdapor Keep, it seems casters are able to avoid taking damage at all during this fight by dropping down the pit before the Demon Wall is even engaged. This also negates the instant death by falling and having no enmity at the same time. It also seems they're able to cast on Demon Wall from below.