Is this even viable? Really high resist?

Too low hp, you will be survivng and not be one shot till maybe ~400-500 floor. At 600 floor with 100k all resist i get hit for several thousands of damage. But you maybe can survive floor 5000. For sure if you are using some items that gives rawa damage reduction for example defiant, pathfinder or maybe alchemy and equivalence mythic.

For pvp, you will die really fast.

Is there a certain amount of resist to become immune to an element?

You can’t be immune to element, only monsters can be. You can be “immune” with that resist on floor 200 but not on 500. If you have enshrined and get blue shrine your resist will probably skyrocket to something like 500-1000k. Mine goes from 4k to 50k.

Dang okay thank you!

Nope. You must have activated the blue shrine which increases ur resist by many folds.
I 've had 400k resist because of blue shrine.

Only in PVE.