Alright, we all know how annoying it is, the wyvern you expect to be a simple mini-boss no worse than the Eye Of Cthulu, ends up being one of the hardest things you battle for a while, my guide will be focussing on how to kill the Wyvern, without preparing any kind of arena and having minimal preparation. Please note this method requires the use of any accessories to extend your jumps, such as the Cloud In A Bottle, Rocket Boots, Spectre Boots, Cloud In A Balloon or any combination of these. It is recommended you battle the Wyvern at daytime for the best results.
Step 1: If you haven't already, locate a floating island and make sure you have easy access to it
Step 2: Make sure you have a fairly decent weapon. A Megashark or a Shotgun is recommended for this battle due to thier high attacks. Do not use a Minishark, this is because while it has a high fire rate, it does very little damage. A phoenix blaster is also not reccomended due to the fact it has a high damage, but low fire rate.
Step 3: Make sure you have plenty of potions, 10-20 potions will be more than enough, Whilst it is reccomended that you use Greater Healing Potions, Healing Potions will suffice if you are careful. Lesser healing potions are not reccomended due to the fact they do not heal enough life.
Step 4: Choose the right type of bullet, Cursed or Crystal bullets are the best due to thier high damage rate. but Silver bullets will suffice. Musket Balls can be used but this is not reccomended due to thier low damage.
Step 5: Go to your floating island and wait for the wyvern, ensure you go up there with maximum health and plenty of ammo and potions.
Step 6: When the Wyvern does appear try to stay on the floating island ground as it comes, and as it sweeps past jump over it, this allows you to dodge most of the Wyvern's attacks, Shoot the Wyvern when onscreen and when the Wyvern is offscreen shoot any nearby harpies. When killing the Wyvern make sure it's head is onscreen, as this is where the loot is dropped.
This method may take several attempts to successfully complete.