The Remains of Kroshius - (45, 35)

Soloable at 60 or thereabouts. The key to killing him is that he's Fearable, so you can mostly just wait 'till he's risen, Fear him, DoT him up, then send your pet after him. He spawns in a nice, isolated bit of Felwood, so you almost certainly won't have any problems with adds from the craters. Make sure you keep an eye on him in case Fear breaks early. Keep a Health and SoulStone up, just in case, and if all else fails, banish'n'bandage.

This quest is easily soloable at 50-53. Be careful, and be patient on this kill:

There are two key spells you will need on this one: Curse of Agony and Fear. I suggest using your imp for the stamina buff, for you might need extra mana via lifetap later if you wish to use corruption as well.

Start the fight, do NOT attack with your imp. Cast fear first, followed by curse of agony, and corruption if you wish. Kroshius is immune to fire spells, so no immolate. Apply Curse of agony and fear as necessary, and when he is at about 10 percent, feel free to nuke him at will.
This fight can take up to 10 minutes, so be sure to remain patient.
When kroshius dies, return to Impsy, and be prepared for a glorius suprise that will be a thing to remember =D

To answer my own question:

Discarding and then getting the quest again is the only way I have found to get your Fel Fire back.

After I completed this quest Niby summoned something he wanted to be an infernal. What he got was El Pollo Grande, a ""Black chicken of death"", quoting from it's description. It was a giant chicken that attacked me, only a level 50 so it was nothing serious. That event was actually quite amusing.

As of patch 2.3, this information is now largely irrelevant. However, this tactic, while excessive, still works, so I'll leave it be. Simply know that there are far easier ways to kill Kroshius, like walking up to him and poking him with your dagger. (Okay, so he wasn't nerfed that badly. You get the point.)

Actually, they can, indeed, be enslaved. I just killed Kroshius not 10 minutes ago using one of them.

A couple notes...

You can only enslave demons that are up to 3 levels above you with the rank of Enslave Demon that you have at this level. If you try to enslave one that's too high, it will say ""Target Cannot Be Charmed,"" leading some people to the assumption that they are not enslavable.

Also, I don't recommend trying the following tactic unless you have Improved Enslave Demon. The reduced chance to have your (higher-level) minion break free during the (long) fight and the extra damage to help shorten it make this worlds easier.

Now for the actual battle...

Firstly, the infernals that you enslave are immune to most of Kroshius' damage. This immunity means that Kroshius will not attack the enslaved infernal unless it is the only thing on his aggro list, and therefore that you cannot attack Kroshius or aggro will be lost on the infernal permanently.

Second, Kroshius will aggro directly to you when he is first summoned, so summon him and be ready to run away or die/Soulstone to de-aggro him. Don't worry, he won't despawn for a while. Now, go and enslave your infernal, making sure that it's at least level 53 (the highest that you can summon at level 50) so that Kroshius (55) is yellow to it.

Third, tell your minion to attack Kroshius and sit back and watch the show. They'll go at it for a while, but not so long that the 5-minute timer runs out if your minion is 53 and you have the talent (I haven't tried it with anything less, but I had about a minute to spare at the end). Your minion is immune to most of Kroshius' damage, as I've said, so he can actually solo Kroshius.

Fourth and finally, wait until Kroshius is one hit from death and kill him. I did it with a Death Coil, just in case it took a second hit.

I hope this helps.

-Aulus

As of patch 2.3 this quest and Kroshius are no longer Elites.

Cakewalk now, been nerfed in 2.3

He's a normal 55 mob (Im guessing he used to be elite), with Felgaurd up he died before my immolate finished!