I hunted in the Rikti War Zone with my fire/kin from level 36 til he was in the low 40's. Then I ran missions in P.I.
You can't go wrong, imo, running 8 man teams.
One last thing:
The main reason that people say that fire/kin's simply fly from level 38 on is fulcrum shift. You see a group of bad guys in front of you, you spawn your fire imps, turn on hot feet, hit the group with flash fires, then hit them with fire cages, jump into the center of the group and hit them with fulcrum shift, watch them burn. Then repeat.
Good hunting!
Currently Playing:
A bunch of toons! (Freedom, Virtue, and a few on Infinity)
Thanks Archantos 
When you see yourself in a crowded room / do your fingers itch,are you pistol-whipped
Will you step in line or release the glitch / can you fall asleep with a panic switch
Since this is your highest-level character, if you're solo I'd actually recommend running story arcs. Team for fun if you want, but the stories are usually worth seeing at least once.
I started the Vanguard arc in Rikti War Zone last night. Having fun. Thanks for the advice.
When you see yourself in a crowded room / do your fingers itch,are you pistol-whipped
Will you step in line or release the glitch / can you fall asleep with a panic switch
My fire/kin just hit level 38 - my highest level character. I've got my 33 month badge but alt-a-holism kept me in the low level game.
Anyhow, I know everyone says fire/kins just sail to 50 after they get FS, but how should I do it?
1) Farm groups together in Eden / Rikti War Zone? Aggro multiple groups in this zone and herd them?
2) Run Cimerora mission maps over and over due to the lack of stuns on the mobs there? Run maps to near completion and then reset them?
3) Continue doing 8 mans?
In the end I imagine I'll continue to do all three since they are fun for me, but I was curious about what people mean whan they say fire/kins go hog wild at 38+
I'd like to get at least one hero to 50 after so many years in game
TYVM
When you see yourself in a crowded room / do your fingers itch,are you pistol-whipped
Will you step in line or release the glitch / can you fall asleep with a panic switch