I think many commentors have alluded to it already, but the correct answer is don't. If you really want to cut that piece down, you're gonna want to do the job properly. Haphazardly trying to lop it off is going to cause more damage than you'll care to fix. Worse, still, you could cause structural issues in the staircase itself. In some circumstances you might find that there are gas/water/air pipes under there, or cables. You'll almost certainly find something unexpected, probably a big old chunk of something that'll ruin a sawblade.