The only times I have shredded cardboard, I used a knife or one of those razor tools.

The only times I have composted cardboard, it was in large sheets under bark mulch as I weed barrier. it rots slow, but that's the point.

I typically burn it or throw it in the cardboard bin at the dump.

I just tear it up by hand right now. We leave it out in the rain and that softens it up.