What is the best way to knock down a big blob of bondo down the fastest?

next to a chisel and hammer would be a mini grinder with a sanding flap disk.

Propane tourch works wonders...peels off like butter. Be smart and don't way overheat so you don't warp the metal but really isn't that difficult. Much cleaner then the flying dust too.

Just don't sand and use an open flame at the same time!

7" grinder with 24 grit then 4" grinder 24 grit and for the deep hard to get to cut a used 7" into a octigon or what ever a stop sign is.Make sure it has equal sides and put this on a four inch grinder to get the nooks and crannies.Remember to always wear you safty glasses.

I use the propane torch and putty knife method. It doesn't take a lot of heat to soften it up.