Nemesis bosses / Crucible - help

Thanks Res, didn’t use blessings so far nor payed much attention to physical res.

The build - skills and devotion points, is taken from a guide by Nery:

I’m well aware my current gear is not optimal but most of the content is easy on ultimate.
Even in Crucible Challenger , up to wave 130 survival is not a problem.
However from that point on, survival becomes very hard on specific bosses.
Is this a balance issue in the game?
I’m not expecting to farm Crucible Gladiator wave 170 at this point but it feels like I’m missing something here.

For example, Father Kymon boss fight is challenging on ultimate even with 80% reduced stun duration and far over 80% fire res.
I guess it’s a combination of low life and low physical res.