I'm looking for a 成语 that means something along the lines of an enemy's best tactic is to hide himself, or to make you believe he doesn't exist.
3 Answers
In 36 stratagems, there's one called Openly repair the gallery roads, but sneak through the passage of Chencang
(明修棧道,暗渡陳倉/明修栈道,暗渡陈仓, Míng xiū zhàn dào, àn dù chén cāng)
Deceive the enemy with an obvious approach that will take a very long time, while surprising him by taking a shortcut and sneak up to him. As the enemy concentrates on the decoy, he will miss you sneaking up to him.
what about "退避三舍", have a look 教育部成語典; of taiwan, in traditional chinese, you can search in english or traditional chinese ^o^
http://dict.idioms.moe.edu.tw/cgi-bin/cydic/gsweb.cgi?o=dcydic&schfmt=text&searchid=CW0000000105
声东击西 and 借刀杀人 came to mind if you emphasize on the hiding part. Making noise in the east while attacking in the west; Trick someone else to kill your enemy without doing it yourself.