We always used a dull saw blade turned around backward, and use a good pair of goggles.
To be ideal I would use a plasma arc cutter to be quicker and look better.
Plasma would be the last thing I would use on metal roofing. When shears do the job, are affordable, designed for the work. That said a razor cutting wheel on a 4 1/2" grinder , works wonders when cutting more than one the same length. Stack three together and geter done!
