I feel you. Over the last few days, I’ve been trying to get a FOI Magehunter to work, too. I went more for elemental damage though, not specific fire damage.
This is the point where I gave up… Heavily inspired by this build of course.
It can do SR 65/66 with some luck (Grava and Kuba are really bad news). But compared to other Magehunter builds (e.g. PRM or AAR), it’s plain bad. 
I really tried a lot - different items, different devotion maps. In the end, nothing really made a difference. You can sacrifice dps for survival or the other way round, but that’s about it. In the end, the result was always just a mediocre build. Conduit is necessary to survive IMHO - so no full Mageslayer set… I got a feeling that FOI still needs a buff to be competitive…