Best way to farm keys

==What is the best way to grind special keys?==
You'll find below some stats I've gathered from my best run. 
I play mono-weapon GS with max skills V2 + full runes so it could be a bit faster or slower depending on what weapon you use.
I also have all rush/return masteries/relics at maxed and I am VIP12 at the moment I write this post.
The important information is the last column "Key per minute" as it's what display the effectiveness of your grind.
This stat stay low early on because you need to reach floor 6,000 before being able to grind for special keys.
Keep in mind that this stats includes rushes too.
What this table show is that, the further you can push, the more effective your grind will be and that for 2 reasons :
* The further you go when grinding, the higher you'll be when you return meaning less time wasted to get to floor 6,000
* The higher you go in stages, the more key you get per 100 stages beaten.
At some point (orange lines in the table), you stop playing online and let the game grind offline for you because you can't be the floor fast enough (or you can't beat it at all because of lack of training). 
 * You entirely rely on rush to grind keys fast enough
 * You need to have pushed as far as you can before being able to push with offline + rush
 * Orange part becomes more and more efficient as you get all specific assist masteries/relics/VIP bonus.
PS : If you're interested to see more in details how the calculation has been done, you can check the spreadsheet directly here: 
TDS Key Grind Calculation
Table capture is from 3rd tab : "2021-01-14 - Mono GS"
Thanks to Pilidria for this guide