Morrus said:
Feel free to "play around" with the wyverns (technically, they don't have to be wyverns, just kinda wyvern-esque with an undead theme going on).
Here is a CR12 advanced wyvern. I'm lifting this from a module, but advancing the wyverns are a very nasty idea. See for yourself. And remember, a rider is technically in every square but I would rule that they get a cove bonus from their mount. Be sure to swoop in, then hover for that full attack next round. Make the archer move so he can't get a full attack out. Cast mirror image on the wyvern if it looks like the archer is about to get a full attack out. Have the wyvern drink a potion of barkskin before combat. Add the fiendish template. Lots of fun can be had.
Bertman
Wyvern, Advanced 15 HD: CR 12 ; Gargantuan dragon; HD 15d12+105; hp 202; Init +0; Spd 20 ft.; fly 60 ft. (poor); AC 20, touch 6, flat-footed 20; Base Atk +15; Grp +40; Atk +24 melee (2d6+13 plus poison, sting) or +24 melee (4d6+13, talon) or +24 melee (4d8+13, bite); Full Atk +24 melee (2d6+13 plus poison, sting) and +22 melee (4d8+13, bite) and +22 melee (3d6+6, 2 wings) and +22 melee (4d6+13, 2 talons); Space/Reach 20 ft./15 ft.; SA poison, improved grab; SQ darkvision 60 ft., immunity to sleep and paralysis, low-light vision, scent; AL NE; SV Fort +16, Ref +9, Will +12; Str 36, Dex 10, Con 24, Int 6, Wis 12, Cha 9.
Skills: Hide +6, Listen +21, Move Silently +18, Spot +24. Feats: Ability Focus (poison), Alertness, Flyby Attack, Hover, Iron Will, MultiattackB, Wingover.
Special: Wyverns can slash with their talons only when making a flyby attack.
Improved Grab (Ex): To use this ability, the wyvern must hit with its talons. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and stings.
Poison (Ex): Injury, Fortitude DC 26, initial and secondary damage 2d6 Con. The save DC is Constitution-based.
Skills: Wyverns have a +3 racial bonus on Spot checks.