The flux makes a huge difference when soldering to aluminum
Check this
This is advertising video with thin alu foil and unregulated soldering iron pre-heated to 500 deg. C or more.
You will not be able to solder Cu pcb to Al host or pill with any flux.
Had seen that video already. Of course, not all fluxes are the same, and as kiriba-ru implied heating up a mere sheet is way different story than a massive chunk of metal.
Check this out, by the way: #49 update on Soldering (batteries, etc) … with Sn42Bi58, Rose's Metal and other low melting solders thread.
Cheers ^:)
Typically aluminium is difficult to solder to due to the very robust oxides that form quickly on the surface. Typically a highly active flux (i.e. corrosive) flux is required (usually very bad for electronics), with a high activation temperature, and a Tin-Zinc or Aluminium-Zinc solder is used. I guess the easier way is to do a hard nickel plating over aluminium, but that doesn't solve your original problem of easily bonding a copper wire to aluminium 
You are right using a solder with a lower melt point is the ideal application here
The Iron he uses in the videos is a Weller WSD soldering station
this station can heat only to 450°C
Getting the heat nessesary is not the problem
Preheat the flashlights head to 150-200°C
I did heat it a bit too much to 250 the the black color changes
If a color change of head is no issue you can heat it to 250°C and just stick the solder wire to head it
Soldering iron with the fattest tip you got
Iron to 400dC should be enough
There is also special SnZn solder which has a melting point at 390°C
I’m not saying if it works or it doesn’t. But that was not just any old flux they were using.
It was Alu Flux from THIS PLACE
EDIT: The stuff ain’t cheap either. $12.07 USD for half an ounce.
I guess that is no more expensive than other fluxes. Heck it might be a scam for all In know. 
Checking the resistance from the tip of the braid to the aluminum body. That “might” tell you how good the connection is (electrically anyway) regardless of the technique or flux…no?