~ This is very easy to get these two achievements.
For the sending a challenge, just send to any of your friends. It doesn't matter if they have the game or not.
For beating a challenge, just sign out of your account, and then sign into one of your silver accounts. Make sure the silver account that you own has your main account added to their friends list. Then, issue a challenge to your main account from the silver. Make the challenge pop darts, and get a score of 1. Send the challenge and then have your main account beat it.
Now you are 50 G richer!
Thanks guys. Enjoy!