Help with hexblast pls

Can some1 give me some advices how to improve my build?

https://pobb.in/w-HY6q-vnb70

I just dont understand what to do next. What do I need to improve step-by-step. My budget is super low so advices like "buy +2 bis wand" are not helpful at all
And can some1 explain to me why almost all(if not all) others r using combustion and herald of ash


Thx