For quick questions, please post in the Help!!! Threads, rather than starting a new one. This could go in Help for Misc or Help for Bondo.
If you want to cut it out without wrecking the visor... Well personally I just wreck the visor and bondo up the wrecked parts, it's a lot faster that way.
If you want to save it though and don't want to rebondo it, you can use the cutting wheels for some parts. I find the width of the dremel's body prevents you from using the smaller thin wheels, so I usually use the wide plastic cutting wheel. That doesn't have much maneuverability though, so it will struggle on corners and small details. You can also try drilling a hole and then using a coping saw, which will give a bit more control and have a thinner diameter than the dremel bits would.
The other method is to resin the pepakura helmet, rondo the inside ONCE with a thin layer. Then it will be strong enough to hold its shape without a visor, but thin enough that you can more easily cut out the visor. Just be VERY careful not to put too much pressure on the helmet as you can crack the rondo. If you wanna be super technical, you could fiberglass the non-visor parts for more strength.
I hope some of this helps.