A tip for killing that Wurstlord.
Find a stage that has a man-eating plant in it and which either has a looped path via which you pass by the plant at some point, or where you can make such a path (such as, for example, using the hammer item).
Ideally you want to be able to get past that man-eating plant without taking any damage but it needs to be tight.
Now, clear out at least that area of the stage free of monsters (other than the man-eating plant(s)) and move around until Wurstlord comes into play. Lure Wurstlord after you and then move around in the looped path and every time he comes near that man-eating plant it'll bite him (if you can get him to move in front of the plant -- mostly he will try to avoid the plants too, which is why you need to be clever).
Once you have this sort of pattern going just go around and around your looped path and let the man-eating plant eat the Wurstlord, at which point he will drop a key. Take that key (no idea what it's used for though).