Back when I made my first "complete" world, I used a mod to spawn some particularly rare enemies in order to get their banners. There's a limit to how much meaningless grind one can stomach, and killing fifty brides, grooms, rune wizards (they were even rarer back then, by the way), doctor bones and such is well past that limit. With the banner changes from 1.4.4, I made another world and got them all legitimately - with some help from AFK farming. If you need an AFK farm to get them, the needed kills are probably set too high for that enemy.