Well here it is, a video guide for the Master Levels in Doom II (Classic). These levels are not easy, I did this playthrough on the lowest difficulty setting for the sake of the achievement. Many of these levels have some absurd amounts of monsters in them and can easily kill you or hinder your progress even on the lowest setting. I do these levels in order from 1 to 20 as fast and as safe as possible.
1) Attack (0:00-1:06)
2) Canyon (1:06-2:50)
3) The Catwalk (2:50-5:17)
4) The Combine (5:17-7:14)
5) The Fistula (7:14-9:20)
6) The Garrison (9:20-11:55)
7) Titan Manor (11:55-14:26)
8) Paradox (14:26-16:29)
9) Subspace (16:29-18:19)
10) Subterra (18:19-20:56)
11) Trapped on Titan (20:56-30:39)
12) Virgil's Lead (30:39-33:08)
13) Minds' Judgement (33:08-34:52)
14) Bloodsea Keep (34:52-37:49)
15) Mephisto's Masoleum (37:49-42:51)
16) Nessus (42:51-43:21)
17) Geryon (43:21-44:39)
18) Vesperas (44:39-47:26)
19) Black Tower (47:26-55:02)
20) The Express Elevator to Hell (55:02-1:01:10)