I suppose that your problem in whirlwinding Nihlathak and his army is that the exploding corpses get you. That problem could be fixed with a single item: a unique ring named Nature's Peace. It has the "slain monsters rest in peace" mod, which means monsters you kill cannot be exploded. That alone could be all that you needed.
As for strategy, if the exploding corpses are the problem, it would be wise to draw the majority of the monsters away from Nihlathak before attacking him. Don't be ashamed to let an act 2 mercenary to help you out, he is surprisingly tough against those explosions, and can be easily resurrected to kill more when he dies. Stepping through a town portal and back would make many of the corpses disappear, leaving you with a safer area with less "ammo" for Nihlathak to use. When Nihlathak is relatively alone, with only a couple minions to protect him, he goes down easily.
You need the nature's peace ring and yes the drop rate sucks. You have a 1/61 chance that a key will drop from each of his 5 drops.
Don't worry about people saying IK sucks. It is a good set to mf with. Use it until you can afford better gear. It works just fine though as a matter of fact you can solo baal with full IK.
Put some Sol runes in your gear for integer pdr. That + 9 iPdr from Nature's Peace should give you about 30. Then if you can get a nice ammy that's 20-25 more. With that gear, in my ~300 Nihla runs I died a mere three times, and out of those, iirc, two were while Cursed and being bombed by a thousand of those red witch balls. :yes:
Also, if you only run Nihla, I'd suggest re-skilling to Berserk or Concentrate. You only really need Ww to whirl through the mobs on your way to Nihla, and in his room. Once I memorized the 3 different cave layouts, I was only 10-20 seconds slower than my teleporting sorc (well, I didn't time it but that's an estimate). Once you're next to Nihla just hammer away. I found Berserk wanting since you cannot leech with it; on the other hand, its damage is much bigger than that of Conc. Your call, and ofc you can always respec nowadays (thank you Blizz for adding after 10 years what should've been there right from the start! :crazyeyes.
Paradoxically, all that safety made the runs so boring that they were one reason that led to me quitting D2 for good. But then I've already played for years and years. You eager youngsters still have some braincells to kill. :nod:
Edit: omg yes, Howl indeed. How could I possibly have forgotten..! One of the best utility skills for a Barbarian (imagine if Sorcs could have it...). In fact that's why I went with Berserk and not Conc: Howl is a synergy and so you get 20 points in it for extra length. With 1-point Howl the monsters barely run a few yards before turning back, so you'll have to howl pretty constantly.