How to solder a wire to a massive copper busbar
Crimp a ring lug onto the wire, and use a screw and lockwasher to secure it to the busbar. You can either tap threads into the busbar itself, or use a nut on the other side.

Soldering makes no sense on a busbar, since you need to mechanically secure the joint before soldering anyway. NEVER use solder as a structural element!

And when you do need to solder large items together, use a torch, not an iron!