what kinda overcapped resists do bosses have?

what sorta ballpark are map boss resists at?
often over 100 no doubt, but how high?

I don't think any boss has overcapped resists by default but they can be if they're under the effect of certain mods or auras. There is no upper limit for overcapping resists, that's why penetration gems are still popular even though monsters have significantly reduced resists now.

I have never seen any monster beeing immune to anything (damage wise)