You should learn what steemmonsters is doing because it is also jumbo size project.

1. You need to draw attention from a huge SP holder.
2. Create an upvote bot that upvote using the some specific tag like #chibera.
3. Make contests and ask them to share your contest.
4. Giving some incentive to the steemians who play your game(You may already did).
5. Make some story contest and let the steemian write.
6. Public your demo of game to public.
7. Invite steemian who has a lots of follower and ask them write about your game.