So I've decided to try mega- Gyarados and run it alongside togekiss for follow me/ tailwind support and terrakion to defeat mega kang/ charizard Y. I'm having trouble building around these core three though. Any suggestions. I kind of want to run choice scarf or life orb mamoswine for ice attacks but I'm not confident in that call. any suggestions?
if you are using mega gyarados and terakion you will need a very good way to beat fairy type. b/c both gyarados and terakion get hit hard by sylveon's hyper voice
You need to run a Pokemon with Wide Guard support for Pixilate Hyper Voice and a physically defensive Pokemon that can draw away attacks that hit Gyarados for super effective damage. I recommend Aegislash and Amoonguss. A Prankster Pokemon would also be good to stop you opponent from statusing your team or setting up Trick Room, Good Pranksters for VGC are Whimsicott, Meowstic-M and Sableye
I would go with Aegislash because it is immune to fighting type. mega gyarados and Terrakion both have a weakness to fighting type so adding heatran would be a third weakness to fighting type. I would suggest using the team weakness chart to help fill in other pokemon it's located in the intro to vgc post on this site.