Whatever it takes.
I've cut 5/8" mild steel with a handheld jigsaw and a bimetal blade because I had easy access to all the tools to do it that way- and didn't feel like setting up a more aggressive/messy cutting option.
I prefer cutting things with a cordless recip saw in general though. It's clean to use, easy to see the cut, variable speed, has a low chance of catching the thing you are cutting on fire, and a low chance of getting crap in your eyes.
I've cut 5/8" mild steel with a handheld jigsaw and a bimetal blade because I had easy access to all the tools to do it that way- and didn't feel like setting up a more aggressive/messy cutting option.
I prefer cutting things with a cordless recip saw in general though. It's clean to use, easy to see the cut, variable speed, has a low chance of catching the thing you are cutting on fire, and a low chance of getting crap in your eyes.