Below you'll find a description as well as both an image and a video of the route I use for the third phase of the final boss fight. Credit to BoursinBurger for all the research, and coming up with the initial route which this is based on.
The big discovery BoursinBurger made, was that when your dynamite is sucked up by the boss, which manhole and even which direction it's spit out at is in fact not random. We use this knowledge to help us blow up the third and fifth target (the ones on your left and right when you first enter the room) without having to go anywhere near them.
Note that the direction dynamite is sucked up from in relation to a manhole, affects where it gets spit out. As such the placement of dynamite at certain points in the route is important, and these cases are underlined in the description below. You can also refer to the image near the bottom to see exactly where you need to place your dynamite.
This route is quick, fairly straightforward, and the most consistent approach I have found. It has some timing to it, but nothing too precise or difficult. There is also some slight variation with the dynamite being sucked up which can occasionally lead to misses and sometimes be hard to identify quickly, but this is rarely a concern.
This entire strategy takes place on the upper level of the room:
1. Go through the crawlspace on the right and place dynamite next to target 1. 2. Quickly run to the right edge of the upper level and drop dynamite off the edge. This will hit target 2 during the short cutscene that plays after blowing up target 1. 3. Run to target 7 and place dynamite down-right of the manhole here. This will be sucked up and hit target 3. 4. As soon as the previous dynamite is placed, quickly run over to target 4 and place a dynamite here. If you are fast enough this will blow up before the boss has a chance to suck it up. 5. Run to the manhole left of target 8. The boss will pop out and attempt to throw a wrench at you, wait a moment and then place dynamite down-left of the manhole. This will be sucked up and hit target 5. 6. Once the boss finishes sucking up the previous dynamite, drop another one off the left edge to hit target 6. 7. Run to target 7 and either place dynamite here, wait for the boss to suck it up, and then place another, OR place the dynamite, quickly run to target 8, and place dynamite here as well to bait the boss into sucking up that one instead. 8. Depending on which approach you used for target 7; run to target 8 and place dynamite, or simply put another one down if you're already there.**
Image for quick reference:
Video for full demonstration:
