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.
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^
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.