Not really a guide, but more so pointers on how I did it:
First and foremost, you will need to do this on a new file, NG+ Keeps the play time from your previous play through as well as your new one. It's best to do all of your practicing on a complete file.
Try your best to avoid the wandering traveler fights on the world map, it's time you could better spend elsewhere.
Practice each level multiple times (obviously). Know where the health is, know the main path and don't stray.
Only do the story mission levels.
Learn the boss patterns well. Know all of their moves, know what to anticipate.
Skip nearly all relics, they tend to be far off the beaten path, and you don't need them all.
The only relics I will suggest are the Phase Locket, which you buy in the village, and the Mobile Gear, which you'll find in clockwork tower.
The Phase Locket will make most boss fights easier if you tend to get hit alot, and is almost necessary for Plague Knight, Polar Knight, and the Enchantress' first form.
The Mobile Gear cuts down a lot of time spent waiting during the horizontal elevator part in clockwork tower, and reduces the Tinker Knight battles to only his second form if you hit him with it immediately after the fight with his first form begins.Addition from Sakori
If you decide to skip all of the relics, you should still have enough time to get this achievement and the "True Shovelry" achievement.
I know it seems crazy, but only visit town to upgrade your health twice.
After you defeat King and Spectre Knight, you get a free meal ticket, and that plus the gold you have (you should easily have at least 6K) from the first three levels will get you three upgrades between the goatician and the gastronomer.
You also get a free meal ticket after you defeat Plague, treasure and Mole Knight, so make sure you have another 6K in gold and you can get two more upgrades which are plenty to get you through the game. This time, use the shortcut trebuchet to get back to town to avoid any wandering traveler fights.
And last but not least, if you die thirty seconds or more after the most recent checkpoint, just pause the game, arrow all the way down to "Return to Title Screen", and select your playfile. If you reset in this manner, it won't resave the time where you're currently at, you will start at the play time you had just before you started the level.
DO NOT HIT RETURN TO MAP! It saves your play time every time you return to the map, no matter if you're returning from finishing a level or rage quitting.
Hopefully these tips helped, and feel free to hit me with an upvote, and send me a message me if you have any level specific questions, I'm glad to help.Addition from Slayer1331fan
Here's a great speedrun video for some visual help. Not my video.