Need some help with CWC magmaorb build

So this is my first attempt to make my own build.
As i like those CWC / COC kind of builds with elemental stuff i decided to have a go at a cwc - magmaorb build, especially with the sire of shards as a nice staff to help in clearspeed.

So far; the build feels oke. Clear speed could be faster but is not bad. survivablity is not bad with a bunch of ES leech (should try to get this up some more for higher tier maps), but overall im happy with the result so far.

But now im kind of stuck on 1 things:
1. Its not really punching bosses. I have a spare 6L in chest i could utilize for some single target action, but i am unsure what would be a good thing to put here for boss killing.

My current build:
https://pastebin.com/5RsVXyuA

So any advice on the boss killing is much appreciated.
Any advice on the build/tree overall is welcome as well. This is the second league i really play dedicated, so i might have made some stupid newby mistakes in the tree or missed some really important things.