Greater Wyverns are a more powerful form of wyvern. Like their weaker counterparts they can poison party members and deal a lot of damage in melee combat. Though not technically immune to spells such as Web and Entangle Greater Wyverns are resistant to them just as they are to all spells. As part of the Tales of the Sword Coast expansion they are designed to be challenging even to high level party's. Greater Wyverns can be found exclusively in Durlag's Tower on the third level of the labyrinth in the bestiary. (See Durlag's Tower Walkthrough)
Though they can be fought in melee combat it may be best to avoid it all together by having one party member distract the Wyvern while the others cast spells or fire projectiles. The decoy would either need to keep moving or be protected magically to avoid damage.
If they are to be fought in melee combat Antidotes and protective spells such as Barkskin, Armor of Faith, and Defensive Harmony are recommended. If the party has a healer it may best to devote them exclusively to healing other party member instead of fighting in any capacity.