There are a couple of firestorm spectres to chose from, but all of the have cooldowns except these ones. They also use Flame dash to close the distance, but when in close range they like to mix in default attack while firestorm is ticking. I was clearing with conc. effect which is not ideal and could be swapped for Awakened inc AoE , but since 9 spectres gives me plenty of screen coverage, i did not felt the need.

My not so good PoB: https://pobb.in/v0BlLlnjQc28

https://poedb.tw/us/Fire_Frenzy

Get them from T17 maps or by taking node called Invasive Adversaries on Atlas tree, which gives a chance for them to appear in any non unique map.