Managed to defeat WY Insanity after about 50 hours, while using Set rule, not abusing items mid battle, not using Amie hax and not overleveling too much compared to Diantha (I ended up with everyone at like lvl87). It was... quite a hard journey tbh. I'm not sure how to talk about my experience with the game just yet, so I'm going to only throw these 2 points:
* The idea of Lysandre bringing his own boxart legendary to the final battle sounds really cool and is a nice throwback to the final battle in BW, but currently the option to use your legendary just feels like a death trap instead of an actual option. Your own legendary would be 7-8 lvl below Lysandre's team, and with no held item or trained EVs (which is rather important for surviving Insanity); WY in particular seems to be getting the short end of the stick here, as Xerneas utterly wrecks Yveltal typing/movepool wise and would just end up using him as setup fodder for Geomancy.
* Looks like the man in Ambrette Town does not recognize the caught Genesect from Terminus Cave, leaving you with no way to obtain the Drives from it. Such a shame too; being able to customize Techno Blast would be quite useful.