These are Rolling magma aka Magma orb spectres.

They are improved version of Templar War Maiden mobs with more HP and much higher dmg numbers.

More spectres that melt my pc nice!

As stated in PoE wiki explosions from rolling magma do not overlap (sadge)

My not so good PoB: https://pobb.in/8q57PS7Lkcuy

For more single target dps chain can be replaced with awakened fire pen.

https://poedb.tw/us/Flame_Creator

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

This video was recorded on standart league.