How should one respond to the Sodium attack? The Sodium attack is characterised by the move 1.Na3, tending to be followed by the fianchetto of the bishop. This opening has never been played by myself, but I seem to encounter it quite frequently amongst lower rated players. What would be the best way to respond?
Play whatever.

Two-three moves like that hardly constitute an "opening". It could help if you told us what kind of set up white tries to achieve in the center (e.g. which of the central pawns are moved.)

Generally moves like that are nothing I'd worry about. Playing just normal moves (occupy the center, develop pieces, take it from there....) should be fine. If you have a favourite black opening against say d4 or e4 you might be able to follow it.

You might want to make the somewhat fundamental decision on whether you prefer to block the b2 bishop by playing e5 at some point or to limit the space for the a3 knight by playing d5.