Grind it when it's the nightfall to save yourself the heart ache. I got all 3 several months ago, but the first I got just from grinding the strike from the map node for it. Took forever.

Any level, although common belief is higher level strike has slightly higher drop percentage. Also, apparently the last flayer killed is the one that drops the cloak, if one is dropped. So, if you want a specific element (solar, void, arc), kill that particular flayer last. I have one, via random drop, and have never really wanted them so I don't farm and have no personal experience, except that I got a solar cloak and I believe that it was the solar flayer we killed last.