Cutting down a steerer tube...

a) attempt to knock the s-f nut down "just enough", and then cut the steerer.

I've always done this and it's not yet killed me.

Have a look at how long the bolt is to work out how far to tap down. There's no need to do it "just enough" – better to do it "enough" if you know what I mean.

I've used a bar and rubber mallet and it moves easily.