Everyone starts with an ELO of 1000, so if you are playing with a boosting partner, you can get Rising Star and Overmind when going towards Grand Master. These must be done in ranked games.
The first person will beat the second person repeatedly until they have an ELO of 1200, unlocking Overmind and Grand Master. The second person will then start winning, unlocking Rising Star, and continuing to win until they have unlocked Overmind and Grand Master. The first player then will need to win one game, unlocking Rising Star.
Note that while the other online achievements can be unlocked in any game type, Rising Star can only be earned in a Classic game.