Reducing the surface oxides of Nitinol is just the first step in getting a good solder joint with this versatile medical assembly material.
Next you have to choose the right solder alloy. You will probably want to stay away from anything containing lead, cadmium, or antimony, particularly in medical applications. And you will want something with a high tensile strength.
The best choice is Indalloy #121 (96.5Sn 3.5Ag). It has a tensile strength of 5,620 PSI and a melting temperature of 221C and is obviously lead-free. It wets well to the cleaned Nitinol.
If you need a higher melting temperature solder (one that can withstand autoclave temperatures for example) you should consider Indalloy #182 (80Au 20Sn) which melts at 280C, has a tensile strength of 40,000 PSI, and has long been considered a highly reliable solder. Additionally, this alloy is available in very fine diameter solder wires to minimize waste.
Soldering temperatures should be 25C to 50C above the liquidus temperature of whichever solder you use and proper cleaning should be always be performed afterwards.