The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

PURELY out of self-defense.
Breaking Bad Season 4, Episode 2

...as a PURELY intellectual exercise.
The Big Bang Theory Season 2, Episode 13

Purely platonic. She's also my maid. Heh.
The Big Bang Theory Season 2, Episode 19

My relationship with Amy
was PURELY intellectual.
The Big Bang Theory Season 4, Episode 3