This post was actually meant to be a reply to a question in one of my previous posts, but I hope some people out there hunting banners could benefit from this post as well.
I spent a long, long time killing Rune Wizards. A couple of things to note: I actually, in hardmode, managed to kill 100+ nymphs and like 90 Tims in the meantime, while I farmed for 50 Rune Wizards. The Rune Wizard spawns in the lower cavern layer, close to hell. If you want to make a farm, you can mine out a giant area with the Drill Containment Unit and proceed to make a platform alike the one I linked a picture of in my other post on this subreddit. To raise the spawnrate, you can make use of jungle blocks (jungle grass or lizard bricks), but just make sure that the top-layer that the monsters spawn on, is not a block with jungle grass or lizard bricks or something like that; It should be stone, pearlstone, regular dirt, ice, pink ice (hallowed ice block) or maybe sand (I have not tested if sand works). If you want to farm for other rare banners like the moth banner, you can use jungle grass blocks as well. Expect to get 100+ moth kill before you get the Rune Wizard banner.
Side note: The spawnrate for the Rune Wizard is very, very low if you decide to go with a corruption or crimson biome, so don't expect him to spawn often if not at all.
I did farm a lot of different banners on the platform i made, and the Rune Wizard together with Doctor Bones, have definitely been the roughest one to get. If you don't have the Rune Wizard banner, Tim banner or Nymph banner, you should farm for all of them in a hardmode world, as you'll probably end up getting the Rune Wizard banner as the last one anyways. I suggest still going for the Doctor Bones banner in a non-hardmode world though.
Obviously you don't want to spend hours upon hours being active in-game when farming for something like this, so you can make a small house WITHOUT a lightsource in it, to have a place for your spawn and a bed to sleep in, if you wish to try and find a blood moon or a solar eclipse to further raise the spawnrate. Battle potions double the spawnrate of everything, so that's a good idea to make use of as well.
The platform I used for most of the days I spent farming for the Rune Wizard banner was split up in two - One with pink ice blocks (for RoDs, Blade Staffs (staves) and bacon (from Pigron), and the Enchanted Sword banner), and the other half was jungle grass. Make sure you have a full jungle biome. It is, of course, indicated by the jungle music, but you can place I believe it was 300 lizard bricks to make sure you have a jungle biome, and then just make the top-most layer jungle grass.
I did change the top-most layer to stone at some point, because I wanted the Rock Golem banner as well (it takes quite some time to acquire as well), but I changed back to jungle grass right when I got the banner.
Last things you need to know: Every other banner than the Doctor Bones banner and the Rune Wizard banner takes no time to collect in comparison to these two. These are the most important ones to get knocked out, and this is the way to do it. The skeleton merchant castrates the spawnrate of monsters, but is sometimes a blessing in disguise, making you some nice profit by allowing you to sell all sorts of loot. If he's not on screen, he will despawn eventually, upping the spawnrate of monsters again.
I used my farm over a week or something, to also earn some money, and I ended up earning around 550 platinum. That is not a typo.
The platform: https://gyazo.com/3152dab826b4b8d9545ba6a4f13d65eb
On the map: https://gyazo.com/aa1fa68294fcfe764f1290c20f49545f
Biome keys: https://gyazo.com/752d2518829ba6ae9542019793e3ff94
(the biome keys from this particular farm is literally all of them with the exception of the Frozen Key and the Desert Key)
This platform is very easy to make changes to, and in the snip above I show the very basic of stone and ice. The banners you see are some of the one's you get an abundance of while also farming for the Rune Wizard and changing around the biomes.
I hope this post helps a couple people out there looking to grab some difficult banners. This will probably be the last banner post I make as they require a lot of work and the reception is generally not that good in my experience.
Edit: As I stand completely still throughout the farming, I use a mix of range and damage, so I use the Terraprisma and the Stardust Cell staff, along with minion damage and extra minions to make the most of it.