15 posts really isn't hard.
1 post in your own introduction thread (potentially 2 or 3 if you respond to other people)
Potentially 1 post in deck help board
1 post in weekly tournament thread stating interest
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1-3 posts in "what deck did you submit to bronze"
1-3 posts in joining other forum events (Guilds have rolling acceptance, accepting a duel, etc.)
1-3 posts in card ideas either with ideas or clarifications on existing card ideas
Remaining posts (2-10 posts) can come from the off topic discussions: politics, sports, movies, books, music, video games, humor, etc. Since only forum games doesn't increase post count.