To earn mastery, you need to complete challenges listed for the Paris mission. These can be accessed at any time by hitting "select" and cycling over the the "Challenges" header. The assassination challenges give you 4,000 mastery each and are typically the easiest way to earn mastery for any level.
The challenges you complete are unlocked immediately upon completion - you do not have to complete the level for credit. However, you will need to complete the level to earn the mastery points for completing the challenge.
I suggest making three save files: one while you're in close proximity to Dalia, one for Viktor, and one when both targets are dead and you are near an exit. Reload your saves continuously to use different methods for killing Dalia and Viktor and once you've completed enough challenges, load the save near the exit to leave the level and earn your mastery points. Your progress is remembered across playthroughs.
Drowning, poisoning, shooting, garroting, and using accident kills to eliminate targets all count as different challenges. By reloading saves, you can easily earn 50,000+ mastery points in about 20 minutes. To max out your mastery, you may have to dip into the more difficult challenges, but you can abuse saves to do this, as well.
- Grab emetic rat poison from the kitchens and meet Dalia while dressed as Helmut. Make a save before she steps away to take her phone call and poison her drink with both poisons. When she gets sick in the bathroom, drown her. Also be sure to shoot her on one of the save files. This will complete 5-6 challenges all worth 4,000 each.
- Grab emetic rat poison from the kitchens again and read the recipe for a Bareknuckle Boxer in the basement kitchen. Grab the lethal poison from Dalia, as well, but use a different method to kill her to conserve the cyanide. While dressed as a waiter, make and poison Viktor's drink with both poisons (on different saves) and kill him similarly to the guide for Dalia above. This will net another 4-5 challenges.