You have many options to do this, I did this in desperation. (When moving and stuff LOL)
Disclaimer: I AM NOT RESPONSIBLE FOR YOU BEING OUTRAGED AT THE BAD CONNECTION YOU'LL PtherROBABLY HAVE. IF AT ANY TIME YOU WANT TO DESTROY YOUR PHONE, XBOX, OR ANY OTHER PROPERTY OR BEING DO SO AT YOUR OWN RISK. LAWS STILL APPLY.
1. Wifi tether your phone into a PC. Learn how to 'bridge connections' (you'll need an ethernet cable). Test xbox live connection and big success.
2. Wifi tether your phone to your xbox using same wifi settings you use on your PC.
Note: I actually did play successful games on my xbox with my connection using my phone, specifically BFBC2 and BF3. Games such as CoD require NAT types to be open which we CANNOT do, thus our phones being unfit to eliminate some lag sometimes. I did manage to play games with no lag and good connection with full reception, however sometimes all I could do was talk in party chat most of the time with no lag.