Maybe two big ass pipe wrenches. Or, one big ass pipe wrench and a big ass set of chanellocks. Really. The easiest way to get one off is this: Get an old sacrificial chain. (put on some work gloves) Wrap chain around big cog. Turn wheel upside down & clamp chain down in a big f#$%^n vise. Now, hit that small cog wit a big ass pipe wrench. Or, a chain whip tool if you got one. You'll figure it out sooner or later. Be good. Have fun.