I really want to get every Steam achievement for Terraria and the next two I want to get is Gelatin World Tour and Boots of the Hero. I have 23/24 of the slimes for Gelatin World Tour and I have come to the conclusion I still need to kill the Crimslime, which is a hardmode slime in the crimson. The world I've been playing on is corrupted. Is the best thing to do is to just quickly defeat The Wall of Flesh with my character I use on my world on a single player crimson world or is there another quicker way to just do it on my world that is already hardmode?
The quickest way to earn that achievement would be to go into a new world, kill the Wall of Flesh immediately, and hang in the Crimson biome until the slime shows up. Achievements are game-wide, not world-specific.
If you want to keep things to your current world though, my recommendation would be to grab enough crimson stone/seeds from your other world, and place them safely in your own world to create the Crimson biome. You will need around 200 blocks to shift the biome. You could potentially make it a floating island so it doesn't interfere with the greater world, and safe enough that it doesn't spread out of control through your world. With enough Crimson based blocks, the world biome should adjust to Crimson, which would allow Crimson enemies to spawn, including your highly sought after Crimslime!
If I were in that position, I would go on a crimson world and kill the Wall of Flesh, then kill the Crimslime. However, if you'd like to keep everything on that world, there's another solution.
When the dryad is placed in a graveyard biome, she'll sell seeds for the opposite world evil during blood moons. So in your corruption world, she would sell crimson seeds, which you can plant in an area that you want to turn to crimson. I would say this is slower, since you have to wait for the Crimson to spread enough to count as a biome, but it is still an option.