You have a misunderstanding about how elemental damage resists stack.

Resists that come from Latents and Awakenings stack additively. So you will need 30 element resist latents and 14 normal resist awakenings to get the 100% resistance against a certain color.

That is why the stock Minerva team is ReMinerva (5%), Skydragon (25%), Skydragon (25%), Lifive (10%), Charité (0%), ReMinerva (5%). The resists add up to 70% and the rest is taken by the 30 fire resist latents.

Now, the leader skill resist stacks multiplicatively with the other leader skill, any shields, and the resist from awakenings. For example, if you take the Minerva team without any latents, you'll have a 70% resistance against fire, a 50% resistance against fire, and another 50% resistance against fire. 0.30 * 0.5 * 0.5 = 0.075

which means that you'll take 7.5% of the damage that you would have otherwise received from fire enemies.

In your case with Mion, you will take 50% * 50% * 75% = 18.75% damage from water enemies.