Recently completed a replay of Marathon Infinity (except the Vidmaster Challenge levels, which I'm still doing. I don't remember the first level very well so it may take several tries).
This game has a seriously fucked up difficulty curve. The hardest stretch of the game ("Acme Station" - "Post Naval Trauma"; also includes the secret level "Aie Mak Sicur" if you choose to play it) is near the beginning of the game. "Acme Station" is like the seventh level of the game. It is usually regarded as the hardest level of the game, and while it's fairly challenging, I was able to clear this level on Major Damage without all that much difficulty by blasting away with all the shotgun ammo I got on "Aie Mak Sicur" (which was incredibly difficult to clear out; I must have died at least twenty times). What makes this stretch incredibly difficult, however, is that they're vacuum levels. And your oxygen gets used up more quickly on higher difficulty settings, a fact that is not noted anywhere in the documentation for the game. And running uses oxygen more quickly than walking, even if you're just holding down the run key and standing still, another fact that is not noted anywhere in the documentation for the game. And you only have a limited supply of oxygen until about halfway through "Post Naval Trauma". I am amazed this got out of testing. It is doable for the very best players on Total Carnage if they do everything just right, but it is also very possible to create an unwinnable save game. As a rule of thumb, all vacuum levels should have a very easily accessible oxygen recharger. (The secret levels "Carroll Street Station" and "You're Wormfood, Dude" are also notable offenders in this category, not having any oxygen rechargers at all). I had to scale the difficulty setting back to Normal for these two levels in order to maintain enough oxygen to beat them. (As I said, I cleared "Acme" on its own fine, but "Post Naval Trauma" wound up being impossible, and I was too lazy to try again on the same difficulty).
Anyway, once you get past that segment the game becomes significantly easier, to the point where the last several levels of the game proper are a joke. I cleared the last few levels on Total Carnage without breaking a sweat. Think I only died once. What is particularly hilarious is that the level "You Think You're Big Time? You're Gonna Die Big Time!" is supposed to contain Tfear's elite guards, and while they certainly are somewhat more powered up than the normal Pfhor, the Fighters and Troopers don't really have anything to distinguish themselves from the other Pfhor except their strength. (The Hunters fire seeking bolts, which, okay, that's pretty elite). The final level, "Aye Mak Sicur", mostly contains Hunters, which are pretty easy to defeat. The only real challenge is the two Juggernauts. Granted, the level is gigantic, so it's still a pretty impressive climax. (Unfortunately, there are large portions of the level that you don't have to visit at all, which is kind of disappointing).
Infinity often gets a lot of criticism for its "difficult" story. However, I think it has the best story of the three games. Once you understand that you're guiding the player through different timelines in order to prevent an Eldritch Abomination from being released by a supernova, the meaning of the game becomes pretty obvious. This is obviously miles beyond what anyone else was attempting in game stories in 1996, much less FPS game stories, and it's a shame the trilogy didn't get more exposure at the time. (Though releasing it only for Mac probably limited its potential exposure). Even now, most people probably wouldn't know the trilogy even existed if Bungie hadn't gone on to develop Myth and Halo. It is a great series in its own right, and while it features a number of limitations of games of the day (clumsy enemy AI, occasional derpy puzzles), they are pretty minor all things told.
Best levels of the trilogy: "G4 Sunbathing" (M1), the version of "Colony Ship For Sale, Cheap" without that stupid platform puzzle (M1A1), "Try again" (M1), "The Hard Stuff Rules" (M2), "My Own Private Thermopylae" (M2), "Kill Your Television" (M2), "Requiem for a Cyborg" (M2), "Poor Yorick" (MInf), "Hang Brain" (MInf), "A Converted Church in Venice, Italy" (MInf), "Son of Grendel" (MInf), "Aye Mak Sicur" (MInf). "Acme Station" would also have made this list if not for the lack of an oxygen recharger. If I had to pick one level from each game, they would be "G4 Sunbathing", "Kill Your Television", and "Son of Grendel", each of which manages to convey the atmosphere it tries to convey perfectly.