For Groundon's moveset, Fire Punch is a pretty decent fire move but Overheat is slightly better. Even though it's
a special move, Desolate Land and STAB make it a really hard-hitting attack. However you don't necessarily need a fire move as Precipice Blades does the work. Also you don't really need Rock Polish , instead a coverage move like Rock Slide/Stone Edge would be better.
For Rayquaza's moveset, replace Dragon Claw for Extreme Speed so it gets a strong priority move. It doesn't need a STAB Dragon move because V-create and Dragon Ascent usually OHKO most dragon Pokemon. Extreme Speed is good since it has priority and is useful for many situations.
For Trevenant's moveset, replace Sunny Day for Wood Hammer or Horn Leech. You don't need Sunny day to increase V-create's power since its already a really powerful move. Wood Hammer does great damage and the leech seed will be able to heal up your recoil damage overtime or you can use Horn Leech to make sure it's healthy.
The Swagger + Foul Play Strat is good but Yveltal should be Physically defensive so it can take care of physical threats as well as defensive pokemon. I feel like this moveset is better:
It should hold Leftovers or a Rocky Helmet.
-Foul Play
-Roost
-Taunt
-Toxic/U-Turn/Defog
Overall, your movesets are really good. I hope these suggestions helped you in any way. c:
