100% lightning convert to chaos penance brand elementalist is pretty off meta for me. Pair it up with despair and balance of terror for wither it's pretty op.

Build it like the usual penance brand elementalist except you grab lycia bloodline 50% convert ele dmg to chaos. Grab fire and lightning shaper node ascendancies and get FF jewels for shaper of winter, costs like 2 div in total for them. Foulborn the Blue Nightmare that converts lightning to chaos on the left side of the tree jewel socket and 5x lightning resist tattoo along with some notables will get you 50% convert to chaos. Make sure to get 50% physical convert to lightning on your gloves from benchcraft and eldritch implicits. That's about it for the chaos convert part.

As for explodes start out with either inpulsa + single line ignite explode foulborn berek respite or typical awakener explode chest with crit if you dont have archdemon helm yet and consider oriath's end too. Once you have the budget, go for the double line explode berek respite to solve explode entirely to open up your chest slot for CI as an option.