Originally Posted by
Cobblemaniac
The best time to get enhance materials for weapons and eidolons is arguably union events. You'll farm hundreds to thousands of those levels and both R and SR materials drop like g̶i̶r̶l̶'̶s̶ ̶s̶k̶i̶r̶t̶s̶ ̶i̶n̶ ̶t̶h̶i̶s̶ ̶g̶a̶m̶e̶ crazy, so much so that you can pretty much max level every single weapon and eidolon on hand if you farm hard enough. Especially if you do union expert cycle memes, but I have a feeling that's dependent on the event since drops seem to differ for every union element.
R (not enhance) weapons serve a different purpose for you, however. You use them primarily to level up weapon skills, not weapon levels, although no reason to say you can't do both, that's double efficiency.