Yanmega's coverage of HP Ice, Air Slash and Bug Buzz/Ancient Power/Giga Drain (this third move is a filler and you can choose the one you prefer) is able to OHKO or nearly OHKO threats such as Landorus-T, M-Salamence and to deal heavy damage to any Fighting and Grass type such as Amoonguss, M-Venusaur, Conkeldurr and so on.
A partner like M-Kang makes sense because Yanmega takes care of its primary threats (Lando, Mence and Fighting types) and M-Kang also offers Fake Out support to give Yanmega a safer turn to get a Speed Boost.
You should't scare away Landorus in Team Preview, you want to attract it so that you can get a surprise OHKO with Yanmega, that's why i wouldn't use bulky Water types or any Competitive/Defiant ability pokemon. Your main problem is facing Thundurus because it can just Prankster TWave your Yanmega, so i would use something like Lum Berry Terrakion to have a strong STAB Rock Slide coverage which deals not only with Thundurus but also with opposing Fire types that may cause some problem to Yanmega.
M-Kang, Yanmega, Terrakion doesn't seem really solid yet (Rotom-W, Fairy types, Cresselia and bulky Water types are still huge problems).
A good core to patch these problem could be a Aegislash/Rotom-Heat/Gastrodon composition: Aegislash is a free switch into Ice and Fighting type moves and it deals with M-Gardevoir and Cresselia. Rotom-H adds some great resistances that you still need (Electric, Fire, Fairy and immunity to Ground) and it completely walls opposing Thundurus and Heatran. Storm Drain Gastrodon walls any Water type not named Ludicolo and opposing Rotom-Wash can't even touch you. Gastrodon's Scald/Earth Power/Ice Beam coverage is also amazing and it is another Thundurus counter (watch out for Swagger lol)
http://www.teammagma.net/teambuilder/?42...39|469|115
Since you have Terrakion and Rotom-H to check opposing M-CharY, i wouldn't use Ancient Power on Yanmega. I think that Giga Drain may be the best option to have a Grass type move in your team (you can OHKO a Mega-Swampert and in general, you have something to hit harder things such as Milotic, Suicune, Politoed and Rotom-W) while Bug Buzz doesn't give you any particular coverage (maybe on Hydreigon, but your Terrakion can outspeed and OHKO it with Close Combat).
Let me know what you think about it and if you would change something! For example, this team doesn't have any Intimidate or Taunt support or a form of speed control.. but i tried to put Yanmega in the best possible position to have a main role in the team.