In anticipation of #JULYGANTIC, I'm thinking of my own experiences with G1-2-3. Such classic modules, so formative of what D&D was to become! Back in the day I ran all three of them, but not in sequence. At the time I was doing a lot of "module cannibalism" and yoinking parts of modules for parallel uses, or reskinning. I know I used all three portions of them at various times - probably all with 2nd edition. Yes, we ran 1e modules without trouble in 2e.
The Glacial Rift of the Frost Giant Jarl saw use, uncoupled from the Against the Giants saga, in fairly true form - the icebound palace of a mighty frost giant. I don't remember why the PCs at the time wanted to deal with these giants (and by "deal with", I mean fatally). They definitely wanted to assassinate the Jarl, possibly for political reasons, and make off with some treasure.
Here's how it went down.
First, you need some context about the PCs. The roster had changed a number of times throughout the campaign as PCs were killed, hung out on the shelf for a bit before rotating back in, or were retired for other reasons. Only one PC was still alive from the very first session: the wizard Zebulon (later dubbed "Zebulon the Cruel" after the Basically Genocide Incident). I don't remember what level he was at the time of the Glacial Rift, certainly 10th, possibly 11th or 12th. Zebulon had been all over the world, visited other planes, got stuck in 20th-century America for a couple days, nearly died many times, quested to the center of the planet, and so forth. He was a wizard's wizard - powerful, with a considerable selection of spells, a ring of flying, and he had an item called the Sunstaff which let him use charges to cast fireball, wall of fire, and so forth like a Staff of Fire, plus some light-related junk. Zebulon was fiendishly clever and stupendously arrogant. Lawful Neutral, but chiefly in the sense that he kept his word once given, and he hated to leave things feeling unfinished.
The PCs arrive in the environs of the glacial rift, and Zebulon - de facto leader by virtue of his level, adventuring experience, and volume of speech - suggests a bold plan. He will enter the rift alone, invisible, flying, (possibly ethereal, I don't remember). He will penetrate deep into the chambers, find the Jarl, slay him, and then high-tail it out of there, emptying his magazine of fireballs and other fire spells on the way out. The other PCs were (rightly) skeptical. It didn't sound possible, or practical, or prudent, and it would leave them on the sidelines. "No no" said Zebulon, "you guys stay out here and take out any fleeing giants once the chaos starts, and waylay any returning hunting parties. That sort of thing." Eventually Zebulon convinced them to let him try ("we get all your stuff back in town if you die down there" may have been a factor).
So in he went, one wizard, alone, invisible. Took him ages to find the Jarl's chamber, even though he could overhear and understand the conversations going on throughout the complex. Once there, he took a look around to see if there was anything he wanted to grab on the way out, and then set about murdering the hell out of the Jarl and his wife. The actual assassination wasn't that difficult, he had no problem pouring out appropriate damage. Obviously the alarm was raised, and Zebulon set about throwing what he could see in the party's bag of holding (which of course had been handed to him prior to the excursion).
Then it was time to book it. Zebulon flew back upward, hurling fire behind him periodically, or chucking a fireball down a side-passage. We did not "narrate" this part - lots of dice were rolled. Because it was important to know how many giants were around, and how near they were to Zebulon, and whether their attacks hit him, and MOST OF ALL did he kill any of them. Sorry, frost giants in the barracks. There were significant discussions about the volume of a fireball explosion. You can imagine. There were further discussions about the integrity of the tunnels. Suffice it to say I was not convinced that the attacks would bring down the whole glacier (nice try, though) -- but lots of frost giants were killed by fireballs, trapped by wall of fire, and so forth. Keep in mind that frost giants do not take double damage from fire or anything like that, they're just impervious to cold attacks. I don't know how many kills he got on the way out, but it was a stack - the last couple spells on the way out, at the top level full of smaller-HD creatures, were particularly effective.
As he flew out of the entrance, bleeding badly and probably at one-quarter his normal hit points, the other PCs unloaded into anything that tried to come out for a number of rounds. Eventually the frost giants stopped probing outward, and the PCs took their opportunity to depart.
Only a handful of loot made it out of there, but that wasn't the point - the goal was to snuff the Frost Giant Jarl. At least, in-character. Zebulon's player knew full well that he was going to reap significant benefit in the form of xp. Yes, the other PCs got a share of it, but recall that in 2e a frost giant is worth something like 7000xp, and there were six players at this session, so with an even split every dead vanilla frost giant was worth over 1000xp to Zebulon. Old-school magic-users must be constantly hungry for big xp scores by virtue of their crazy xp table. The sortie was absolutely worth it.
"That was insane, I can't believe that worked, Zebulon. You're [expletive] crazy."
"Perhaps I am. But the Jarl is dead, and the rest of them will be in disarray and mourning. Easy pickings for when we go back tomorrow."