I believe what he was trying to say was that without a spawner, he'd have to wait around the Nether forever. (Although he doesn't seem to have taken your grinder suggestion into account.)
Anyway. By your math, getting three skulls takes... 25*3=75 withers.
No idea how often you can find wither skeletons just running around, though, especially if you have to fight off blazes in the meanwhile.
The reason I asked about the Mystcraft fortress page was that it would probably be easier to set up a grinder without having to worry about ghasts or lava (or pigmen, for that matter). It would be tricky positioning it entirely over the fortress, though...
Heh, you'd be surprised how fun that can be.
What I did last time I was doing this, I refurbished the fort as a quake map
. Added some areal faith plates, obsidian plates dispensing heal splash potions / arrows / portals and to be honest, I was quite enjoying the experience.
I'd go a step further if I did it now though. I'd make a special MPS armor just for this challenge map, spread a few charge pads with very slow generators beneath them
. Would be fun.
Heck, I was considering making a mod which'd allow for making arenas like this with custom drops and quake weapons, and scorekeeping and stuff. Damn, that would be a blast.