Against Marth, Mario has SHFFL Up-air to Forward smash on stage, but for the most part from what I've found, yeah, an offstage/dropzone back air usually does the trick. I usually do that, or throw fireballs. Typically if the fireball catches them from out of a side B or their jump, it does the trick. Or you can just style on them with a cape when they Up-B. It's difficult for Marth to sweetspot properly on BF if they don't have the timing right. (Note: Cape does not work well at all on Marth on Yoshi's).
You can also edgeguard Marth with the Marth-Killer and punish the recovery with a Forward Smash (see A Rookie's video on this) or a Down-smash or Up-smash, depending on the percentage.
To cover spacies' illusion and you have them offstage, if they're recovering from above and want to get the platform ledgecancel or simply recover on side platform, a FH fireball from centre stage can catch them. (The fireball should bounce once on the platform, and then makes its way offstage) Make sure that you do it early so they drop down and get hit, preventing them from getting on side platform. If they do, then you have to punish and adjust accordingly. If they're recovering to the ledge, I find that fireball is a suboptimal option. A WD F-Tilt would be better, but if you wanna go for the fireball, then throw one on the ground, once again making sure it hits the stage once, and then punish with an off-stage backair.
I wish I could show you an example, but I have no way of recording and uploading it to here. Hope this helped dude.