A good old fashioned 1KW soldering iron would help a lot. And a lot of patience. Don't apply heat directly to the FET tab, apply it to copper near the tab. That will help ensure the FET doesn't overheat before the copper reaches a high enough temp.
If the busbar is large, then it may be too difficult to solder it, regardless of your method, without destroying the FET. If that's the case you'll want to solder to a smaller piece of busbar, then bolt it into place to connect to the rest of the busbar.