There are a number of ways to cut flexible metallic conduit (FMC), or cables made with interlocking armor, such as MC, AC, or old style BX.
The first thing to do is take a look at the stuff and see that it's a spiral construction - the edges of the spiral strip interlock. You want to cut across the spiral strip.
This illustration shows the cut you want on old style BX cable, but the idea is just the same:
The simplest, fastest, most consistent way is with a specialty tool made by Seatek, the Roto-split:
The screw on the underside of the tool clamps the armor in place. The crank handle turns a little circular metal cutting saw blade that cuts the spiral. There is a set screw that adjusts to limit the depth of cut. You squeeze the handle to press the wheel down, crank, and stop when you get through the high part of the spiral strip.
You don't have to get all the way through the spiral, if you get most of the way through, you'll be able to untwist to separate, or bend the cable to pull apart at the cut, and untwist.
You can make the same cut the rotosplit makes with a hacksaw, but it's a little tricky holding the flexible cable in place while you make that cut. But in the OP's case with just one cut to make, you could hold the piece in a vice or otherwise clamp it and carefully make the cut.
Another method works with nothing more than a pair of diagonal cutters / dikes.
You bend the cable just sharply enough that the interlocking edges pull apart, then untwist to further separate the strip. You can then clip the strip where it's pulled apart.
This is fast but a little sloppy - the armor will be a little mangled, you'll need to tidy up the cut ends by trimming off the mangled bit. Luckily you'll have the right tool for the trimming handy, same set of dikes. Personally I don't like this method with MC or AC cables, because occasionally you'll nick the insulation of the wires inside. If I have to do more than a few cuts, I get the Roto-split.
No matter how you make your cut, the process leaves a very sharp edge on the metal close to the wires - too close for comfort. The connectors used with FMC / MC / AC are often designed so that once the fitting is in place the wires don't contact the edge of the armor. They also make anti-short bushings, little plastic split bushings that slide on the end of the armor:
Some consider anti-short bushings optional or unnecessary, I am in the camp that considers them required every single time, unless you're using screw in connectors, which are no longer popular, but provide the best possible protection for the wires: