Hate to be the bearer of bad news, but, your only hope is the GTS. Since X and Y came out in 2013 (and I hope you're talking about X and Y since your hashtags say xy) and Genesect is a mythical, all the mythical events for X and Y have been over for about 4 or 5 years. But, just to help ya out, I'll tell ya how to get some mythicals easily from the GTS.
1. Get a shiny Pokemon you HATE (for me, I used Relicanth) and get it up to lvl 100 if you haven't already
2. Put that shiny into the GTS and ask for any mythical (for me, I used Meloetta)
3. Wait awhile and your mythical should arrive! The wait time should be around 10 hours to a day, but no more than 3 days