How do I get the wyverns to spawn?

[1Step2]

I've been going across my world using the gravity potion but I have yet to see one. Yes, im on hardmode. Its on expert and I really want those souls of flight. Same with the Sand Elementals. I go back and forth during an sandstorm but yet to see one too. I got plenty of Ice Cores but I use magic wepons way more then melee and ranged.

[Tiff]

I stood on a floating island for awhile killing harpies (trying to get the giant feather) and about every 2 minutes I'd have a wyvern swoop in. I also had a water candle on the island. Might be worth a try.

[Ozionian]

I'd reccomend making a skybridge, and use a Battle Potion. Now, walking across it, using low speed on the Mechanical Minecart or Medium speed on a regular Minecart. Or you can just walk/run along it, or ride a Unicorn along it.
It's not perfect, but I did this earlier when looking for a Martian Probe and I ran into 3 Wyverns, one after another.

[AlphaPlum]

Just sit on a floating island with a water candle, use a battle potion for better odds of a spawn, for sand elementals its the same thing

[1Step2]

Thanks guys

[Seraphita]

Just for the record. Those "rare" monsters don't appear instantly. You must be there and survive whatever that attacks you for a short while then it will try and come at you. Sometimes it's quick, sometimes it's not but they should spawn rather fast as long as you stay in the area.