Word/phrase to describe enlisting your enemy's help to succeed?

Solution 1:

It sounds like you need to describe a situation where not only has your enemy become an ally, but the alliance has been formed to defeat that same enemy.

The necessary expertise is solicited by drawing on the best source of knowledge about your enemy's tactics.

Perhaps "keep your friends close, and your enemy closer" would fit here?

Solution 2:

Sell your soul to the devil is defined by McMillan dictionary as

to do whatever is necessary in order to get something you want, even if it involves doing something dishonest or immoral.

Or perhaps, make a deal with the devil.