use the silver blade, vs.1 use the heavy style, vs 2 use group style. if you see them rearing up to attack, dodge backwards.
Its also best to fight them on level ground w/o too many trees around, you can easily lure 1 back away from a group of them to do this. They are also stunnable, so you can get an easy 1 hit kill on them.
If you're patient and tactical wyverns are easy even on the hard difficulty.
This is also assuming you've put some points into the silver blade to get the upgraded combos, do additional dmg and what not