This achievement is pretty straightforward. The good news is that the game has a total of 112% total progress, with 100% being from the original PC base game and 12% coming from content that was initially added to the PC version via DLC/updates, all of which is included in the Xbox One version. You only need 100% on your progress, meaning there is wiggle room in terms of what you have to complete. 20 hours is extremely lenient and should not be an issue as long as it's not your first playthrogh of the game.
I did this on my second playthrough and combined it with the 5 and 10 hour speedruns. I mostly followed this awesome guide from Steam
for the speedrun, doing the bare minimum as quickly as I could. Once I had completed the game in under 5 hours, I went back and worked towards 100% progress, then completed the game a second time and this achievement unlocked at the end of the credits during the summary screen.
After destroying the three Dreamers you can get into the Temple of the Black Egg in the Forgotten Crossroads. Interact with the statue next to the bench and your completion percentage will then be shown in your menu and on your save slot.
For progress towards 100/112%, this is what counts:Upgrades/Pickups
(1% each charm = 40% total):
(1% each = 17% total):
(1% each = 7% total):
(1% each = 3% total)
Colosseum of Fools
(1% each = 3% total
(1% each = 4% total0
Since you don't need everything for this achievement, I didn't bother with the Pantheons or the delicate flower trip for the last mask shard. I also was missing a vessel fragment and only did the first two Colosseum battles as the third is annoying. Once I was at 100%, I killed the Hollow Knight and the achievement unlocked at the end of the credits (my game time was about 11 hours so this is a pretty lenient time requirement).The Hollow Knight wiki
is a great source of information on anything specific.
These two maps helped me immensely, as well:Basic mapFully interactive map