I believe Mercy Stroke one of the good cross abilities that can be used, out of GCD, 200 potency, doesn't break your combos and heals you if you were lucky to land it correctly.
But, I believe it would be more beneficial to its users if the heals was somehow much more than a lucky strike in order to get its full effects.
So, as an improvement, I think Mercy Stroke would applies an effect that would guarantee its user the heal if the target dies within 3/5 sec of its application, in that way this ability would be really helpful in many situation, especially tanks when it comes to handling adds or so.