The Corpulent died in about ten shots, maybe nine. I lured out three more Venom Lords one at a time and had little trouble handling them. Another Corpulent, and already Ember's body count has reached six, with no vibrations on the "Oh No" meter indicating tactical difficulties. Then...
Then I had to fight three opponents at one time, and the meter was going ding-ding-ding!

One opponent was routine, two was difficult. Three was a real chore, requiring lots of mousework, taking lots of time. I'd score hits on one target only to have another race up, force me to tempt, then run, shoot, but here comes the first target again. Not a moment's peace, more running, less firing, some missed shots, and the health regeneration of standard monster types was definitely running up Ember's tab. Each extra target added to any given battle doubled the workload, cumulatively. How many could Ember handle before it became too much and the regen factor overwhelmed her damage rate? I didn't know yet, but if I had to bet I'd have guessed five. That meant that any boss packs I encountered might not be manageable with standard tactics. I'd either have to find a way to separate them, to deal with just a few at a time, or I'd have to find some way to park the whole lot of them that still allowed me to progress.
Slow, patient play, in classic Diablo 1 style: take a single step (or what passes for one in D2), wait to see what comes charging out at you. Work your way forward one inch of ground at a time and set maintaining control of already cleared territory as the second-highest priority (right behind not getting killed).
Ember had a small cushion to the right of the gate, then worked left, soon found the corner, and started clockwise along the edge of the map one careful step at a time. Turns out that corner wasn't quite the real corner yet. Down one segment, the map turned outward again and Ember woke up a pair of Doom Casters on three separate occasions. This, the true corner, turned out light on the Venoms. Kill a few regurgitators and clean up some of the cornered Doom Casters and Ember had herself a bona fide Loop Track. Check the map overlay in the shot below to see how much progress Ember made in the first half hour.

You might be surprised to learn how shockingly little ground would be gained in the next half hour, but remember that Ember did not kill all the targets in the area cleared so far. In fact, more than a third of them had been led elsewhere and remained a threat, more so even for being all bunched up. So there had been an artificial surge in her ground gained by way of a Portal Park maneuver, but this didn't actually remove any threats. It only redistributed them, and there were limits to that. Each parking would tie up a rather wide region, which if approached even at a wide range would draw out all sorts of rabid beasties in larger and larger mobs. The Portal Park was Ember's Ace in the hole, but if she overplayed her hand she'd lose for sure. Once she had moved beyond that initial range, progress slowed. A lot of Corpulents in the corner region made that a quick clear, but now Ember was dealing with mostly Venom Lords and some Casters, and progress was just CRAWLING.
Here, for the first time, Ember had to deal with more than three at once:

I stuck to the corner and looped around counterclockwise in that space. Five minutes this lasted, then ten, slowly ever so slowly wearing them down only to watch their regeneration erasing my gains. Two steps forward and one step back. Two steps forward and one step back. One step forward and two steps back! Ack! Ack! Ack! ... Two steps forward and one step back... Two steps...
And then there were three:

I had to brighten that picture to clearly show the third, mostly off the screen in shadow. Another three minutes passed. Two steps forward and one step back! Ack! Ack! Ack!
And then there were two:

Two was the magic number. Ember can handle two. This was the final lap. For then there was only one, and he too was almost done:

I got the final kill with the body of the previous balrog still in sight (brightened slightly to show through the shadows).
All right, recap: fifteen minutes spent on four Venom Lords. Ground gained: two steps. That's not even as far as Ember can throw a potion, and let me assure you her throwing arm is not particularly strong!
On a little farther, kill another Venom, then another, and then just like that we begin again with the local barbershop quartet:

Does two pair beat four of a kind? Nope. Not in poker, and not here in Hell. This was still a drawn out fight...

...but I was able to wound those Casters eventually, then handle the two Venoms in short order. I made sure the Casters would run into the corners, not back the way they had come.
A few more smaller battles with one or two at a time... I look up and over an hour has passed. Here's the situation at that point:

To the left of the broken white line I drew, lies the territory under Ember's control. You can see that I've ventured farther east by now but all that area is still in dispute. The solid green line marks the path taken by Ember on her initial Portal Park maneuver, culminating at the yellow X, where all the critters awakened on her way there were now gathered, arguing over who ate the sorcie without sharing with the group.
Now I could have tried to secure some of that uncharted middle ground, but I was hoping to find the stairs soon. I'd like nothing better than to get out of here without any more serious troubles.
I'd also like a $100,000 advance on a book deal, a winning lotto ticket to arrive in my mail box (I never play the lottery), and an intelligent woman I can get along with ask me out to dinner by the end of the week. Wanting things is good, but not always an indication of actually getting those things.
I worked a bit more but soon ran into a crisis: seven Venom Lords and two Casters woke up all at once and closed in on Ember from two directions. I knew right away what creek I was up. Without hesitation, I made the decision to Park this crew and off we went, due north into the heart of the Steppes, then turning more toward the top of the screen, moving in a straight line toward the yellow X (which was not there on the map, of course, but I knew the spot). Then it occurred to me in midflight that there was already a mob there waiting and that I'd best not go all the way there. I turned back toward the Pandemonium Fortress, looking for a way to work around behind that spot, up along the north edge where Ember had passed on that first maneuver. If I could get this group over there, I'd have one Hell of a mass of monsters in roughly one largish parking lot, but they would be out of the way.
And then a chasm rose up out of the depths of Hell, reaching out with taloned hands to grasp and trap poor Ember in almost exactly the same way she'd gotten trapped by a mass of Leapers in Act Four Normal.
Uh... someone forgot to tell me that was going to be there. I've got half a second to get a grip on myself and figure out what to do or this is going to be over right here, right now.
Find Out what happened next.