Crampons are tricky things to hold steady while sharpening the points. Trying to hold them down while filing is awkward, especially when you want to file accurately.
I put a small machinists parallel clamp into an engineers vice, keeping the moving side free so I can change points. These clamps where a standard apprentice project back a bit so not too hard to find secondhand. They are a very handy and strong little clamp. Another option would be to use vice-grip pliers, again held in a vice or clamp to keep it steady so you can use both hands to file accurately.
You can rotate the clamp in the vice to get a good angle; it worked pretty well — less filing effort, more accuracy, and way less cuts!