I think this is viable, and I'm testing. It's clvl 98->99 viable running Nihl, tested that also. Swap Phoenix for facet'd Stormshield (go for max block). It's cheaper, protects you, and allows you to tele-stomp; skills Firestorm/boulder/fissure/volcano/Oak (for Nihl) or HoW (for AT) maxed, one in bear and go for it. Don't need FCR ammy; Mara's is better. Merc for AT is might Merc, /w Infinity, Fort, IAS'd Andy's. Nihl is HF: Infinity, ias'd G-Bane, and Delerium, last of which makes things confused and keeps them walking into fissures.AT Fissure Druid (sub level 98 leveling)
Method of running: get to waypoint, buff BO (I'm not sure is battle command needed, it won't really make difference except for maybe HC players, but you can also cast it when you are killing some bosspack and have nothing else to do) then summon oak and enter AT. There you focus only on bosses and ignore champions unless there are like 4 at a bunch. Also ignore Invader packs with only 3 minions most of the time unless they come with other packs too, they take too much time and offer too little for it. Cast Fissure then you can play with Grizzly (lol) or you can tele on top of boss getting ready to firestorm him as soon as timer is up. Do not underestimate firestorm, it is killing very fast. Fissure is better on many targets naturally. Volcano is also good option for Invader boss. Naturally skip fire immune Invader boss.
What you really want is Infinity on merc, and as much -enemy fire ress as possible.
Ravenlore (facet), Enigma, fcr amulet (19+ is ideal), Phoenix shield, Hoto, Mesh belt, Magefist, dual beta BKWB, War Travelers (you can go with any other boots, I went for a bit of MF). 8 skillers, cube, small charms.
Overall, idea is to have nice - enemy fire ress and 99 fcr breakpoint. First of all, 99 fcr breakpoint is not needed because your spells are on timers and you will just wait most of the time. So lower fcr is very nice too. Lower breakpoint is at 68 fcr, and I even think lower than that can be good too, but I'd stick at least with 68. There are other possible equipment options, but this presented here I find nicely balanced. Phoenix giving life and mana is nice bonus too. Naturally, merc is using Infinity. Mine uses Fortitude and Vampire Gaze with it. Merc is might, because you need something to kill Invaders faster. They are quite tough even after - fire ress from equipment.
Leveling 94-95: ~8400 xp/s, 6 hours 50 min to level up. Comparable to 223 s Baal, and 200 s Lister running.
For comparison, I've done leveling with Pit windy, with, if I recall correctly, something like ~5k xp/s. I just know how much faster AT Fissure druid is than Pit windy, he is even comparable to Pitnec in leveling which is great in my book. But I don't think AT Fissure druid is best possible option for leveling. Windy running Lister is quite safe and quite efficient as well. He needs 3 min 20 sec Lister running to be comparable to Fissure druid according to my tests, and I do believe he can do it. I'm not sure would random player have faster Lister runner or AT runner. Fissure druid requires some positional playing, so I'd say Lister windy is possibly better option to level up the druid for most players.