Thanks, happy to hear it is to your liking^^
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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.
1. Currently there is one companion more. It is unlocked through the right Perk Tree and after unlocked appears in each run and can be recruited. The perk also is star shaped so its easy to spot.
2. Is kind of a puzzle where you need to think outside of the box. For a hint: You need to have unlocked the blacksmith fields from the right perk tree and artefact smuggler from achievment unlocks. It is a more late game thing.