Recently beat Descent on Insane difficulty with a rule of no mid-level saving allowed. Maybe it's just because I learned what to expect about surprises around corners at some point, but I really feel like the middle of the game where it completely swarms you with homing missile-launching Super Hulks and hitscan insta-firing class 1 drillers is actually the hardest part. I was constantly struggling to make it to the next level with a good score bonus to earn just a few extra lives to survive the next stage. In the final stages, however, I was running around with 10+ lives across levels until the end.
It's unfortunate, but gitting gud at Descent is really about either engaging enemies so far away that it doesn't activate their aggression or having foresight about the level design and making liberal use of homing and smart missiles to kill enemies around corners where they can't shoot. It's a shame this game didn't have some more fair enemy variety, the map design gets very cool in the later levels.
Moving on to Descent II now, and it's amazing how much of an improvement over its predecessor it is. More interesting enemies, more weapons to tackle them in different ways, and the map design gets balls crazy already on the 2nd stage in the game!