For mastery of each weapon, I went to configuration and turned on show weapon mastery under the combat display settings. This will allow you to see the % meter for each skill just below your health.Best done on Prince of Darkness difficulty as the upgrades will help with the challenges, and they reset on NG+
I had chosen a wolf portal (in the city) that was next to a magic font, so I could fill up my magic fairly quickly. You could also use ensnared demons if you have the xp to spend, as it would be much faster.
In the Kleidos challenges, under recollections of ruby, is the challenge Infection: second seal. In this challenge you will face 50 possessed citizens. I would just run in and kill with different skills until I died. Dying here will let you restart the challenge with the same health, but the magic you used is gone.
Using this method, you do not need to run around the world, looking for enemies and magic fonts. Combined with the shop's proximity, this achievement will be done in no time!