Staff/Invul build - I think I'm missing something

[Kakapunt]

Hello All - after trying this on a tank and finding the damage less than appealing I decided to try re-building it as a brute.  Earlier iterations of this build had the defenses in the mid 30s and I felt that Invincibility would account for the short fall, but all the builds for /Invul I've seen posted tend to try and push for the softcap?

 

I think I'm missing something obvious here; I'm sure this thing can be tougher.  Many thanks in advance.

 

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