Damien Darhk vs Ra's Al Ghul

And their organizations.

Darhk has his powers.

There must be some reason Darhk was on the run from Ra's, despite his powers..

There probably was a good reason. Then again judging how Damien reacted when the source of his power was stolen I don't think he and sanity have been in the same room in a super long time. Based on feats he should kill Ras.

Strength of numbers, maybe? Darhk only has two hands, and seems to need one per person.

Yep, and from the way Malcolm talked about the league of assassins..they wielded a lot of power, probably more then the hive.

Well, Thea showed resistance to his abilities after being revived by the Pit. Maybe over a century of constantly being healed and kept young has given Ra's a similar kind of resistance? I really have no idea.

Without his powers, Ra's would absolutely murderstomp Damien though. Darhk got his ass kicked by a pair of prison thugs.