Yotsuya has an unusually high concentration of Mexican restaurants. I ate at one earlier this year, which was pretty good, but I don't remember its name or its exact location... I just know it was not one of the following:

http://www.bento.com/rev/1492.html
http://www.bento.com/rev/1748.html
http://www.hotpepper.jp/strJ000032618/
http://www.hotpepper.jp/strJ000748134/

Perhaps the best strategy is just to go taco-hopping from Yotsuya Station west toward Shinjuku until something sticks.