As you know, I got my Ancient Petrified Leaf last Friday. I knew going in, that the quest was going to be extremely difficult, but not for the reasons that have transpired. Since I started playing on Burning Legion, I convinced myself that Alliance were self centered, 12 year old, assholes (as are Horde if you are on the other side). I did this to save my sanity on a PvP server. I knew ganking would happen, I knew I would be camped heavily, and I really didn’t mind at all. It’s bound to happen. Now, since I started my epic quest, for the first time in my entire WoW “career”, I am furious. I can’t count the amount of times, an Alliance rogue has popped up in the middle of no-where, in FULL Tier 2 epics, to despawn my demon. If this was some rogue in just normal blues, I might think that he was there to farm, but a rogue in full tier 2 has NO BUSINESS in the areas I am in, but to grief Hunters doing their quest. Ugh…. ok, thats off my chest, now onto the demons themselves.
Klinfran - Burning Steppes
This guy was fairly easy, and thus, was my first demon kill. The basic strategy, is to hit him with a Scorpid Sting when enraged, and he will only hit you for 1 damage in melee. This is the longest fight of the 4. The rotation is Scorpid Sting, go into melee, hit 3 or 4 times, Wing Clip, run out to range, Scorpid Sting, rinse repeat… The fight took me about 7-9 minutes, and I took him out in 1 shot. Woohoo, off to a good start.
Simone - Un’goro
I had heard that this fight was one of the easiest to do, so I tried it second. The basic strategy here, is to take this demon’s pet out of combat so you can focus on the demon herself in melee. Being an engineer, I had an advantage in that, once the pet was out of my Freezing Trap, I could use my Goblin Rocket Helmet to disable it for another 30 seconds or so. My first couple of attempts, I could only get her to about 7-10% before having to Feign Death at around 1% health. So, I went to the auction house, bought a Nature Protection Potion, and a Demonslaying potion, and with these, was able to easily take her out, and have enough to even kill the pet easily 
2 demons down, woohoo!
Artorious - Winterspring
Ok, so I have only seen this demon spawn once since I have had my leaf, and this is where my frustration comes from. This is where the Alliance rogue despawned my demon on the 3rd try, thus forcing me to wait another 3 hours for it to spawn. I have yet to kill this demon, so I will update this once I do.
UPDATE: Have tried this demon twice in the last 24 hours, and have had horrible luck with him. Got him down to 6% last night, and 14% this morning. I have no doubt that he is going down when I get home from work (as long as he is up).
UPDATE 2: Took this demon down last Thursday Turned in the quest as fast as I could. Expect an update with screenshots soon.
Solenor - Silithus
This is by FAR the hardest demon of the bunch. I won’t even go into the amount of preperation needed for this demon until I have succesfully killed it. Needless to say, I have spent in excess of 50g in potions so far trying to down this demon. More to come soon.
UPDATE: Took this demon down on 3/22/2006 at around 9:00pm. Used 2 Greater Shadow Protection Potions and had a priest buff me with Shadow Protection for a total of 91 Shadow Resistance. I resisted about half of his fears, and only took around 1k damage total.
Good luck!
I have a guildie that kept having the same problems you are having