Mercy Stroke is also off of the global cooldown and can be used between abilities. It will yield a dps increase so you should have it available.

One downside is that it has a long cool down and can only be used below 20%, sooo there will be some boss fights that you can only use it once. If a fight has adds its likely you can use it on adds then again later on the boss. There have been some fights that I have chosen to go with an additional defensive cool down in It's place... This will have to be determined on a fight to fight basis, but it is a dps increase.