Need advice to improve single target on my CwC Cremation Necro

Hi,

So first of all I know Cremation is not the best way to go (VD Spellslinger seems all around better) and on top of that, Creamtion needs a second setup for bosses (like Armagedon Brand).

But humor me and just tell me how I could improve my single damage on bosses without respec.

My clearing damage is good enough (simu, t16 maps are not a problem) and bosses are ok as long as there are enough adds around. But lone bosses are hard because they last for a long time and so sooner or later I make mistakes and die (it doesn't help that I don't know the fights well). I generally do Conquerors deathless but always die during Catalina and Elder and never managed Sirus and Shaper yet.

I tried just switching greater multiple for fire pen for bosses but the extra damage doesn't make up for the loss of number of projectiles.

https://www.pathofexile.com/account/view-profile/Schukala/characters

If anybody could have a look and give me a piece of advice that is not too expensive (I can't afford Awakened Greater Multiple Projectiles for example) I'd be grateful!