Since it is mostly rng what tf items you will find its hard to say.
Fastes way would be to just take any tf item you get to accumulate mastery across all forms. But 400 isnt that much so it will come naturally over time. Winning a run also gives bonus mastery for all unlocked forms and there are a few artefacts which boost tf mastery gain you can find if you're lucky, like the Mastery Ring Relic and Demon Ring cursed artefact.