Many "weak" Pokémon have been given stat boosts, new abilities, or updated typing to make them viable.

Grab the patch, fire up your emulator or console, and get ready for the toughest version of Kalos ever created.

As long as you are downloading the (which are small and do not contain Nintendo's copyrighted code), you are in the clear. Avoid sites that offer "Pre-Patched Wilting Y .3ds" files, as these are often hosted on untrustworthy sites and infringe on copyrights.