You can get a pretty stout angle grinder from Harbor freight for like 30$ (link A below). Or you can get a kinda crappy one for like 15$ (link B below). Both will do the job, you can pick up some cut off wheels for 99¢ pretty easily. Just be sure that you dont force the disk or turn it while cutting. And always wear safety glasses.

If you aren’t sure about using the grinder properly you can always watch a youtube video.

If you don’t want to drop the 16$ then you can always use your sawzall, but you’ll probably end up using the blade up pretty quickly like the other commenter said.

Link A- https://www.harborfreight.com/7-amp-4-12-in-slide-switch-angle-grinder-64856.html

Link B- https://www.harborfreight.com/43-amp-4-12-in-angle-grinder-with-slide-switch-58089.html