With Infernal Dawn is around the corner, have you made any preparations yet?
My guild had began farming T2 Fire and Earth RR. Though its time consuming, it is a good way to save your ISS.
There is no news yet about how much resist you should put into your cores. Therefore, it is good to assume that you need to put all fire essences in it just in case.
There are other options available if you do not wish to waste your time farming those essences from raid rifts, for example, buying them straight off the vendor. Currently, there is a new world event vendor that sells them (see the picture below).
Its located at the corner of the world event place. The top of the goods cost 20 ISS and 500 molten gold ingots (new world currency). It gives you 19 fire resists with the outstanding stats along.
This new planar essences arguably the new BIS for certain callings, therefore, its really a worth item buying.
note: dont forget to show the filtered items so that you may buy the other planar essence that suits you.