...you don't?
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It requires the enemy to have less than 20% HP left, and has a 1 minute 30 second cooldown. Until the enemy is under 20% it's useless, and the cooldown is 50 seconds longer than Assassinate, which is a 40s cooldown for the same potency (minus the healing if it kills). When it becomes useable, you get maybe 1-2 uses on a boss and that's it. So yeah, I find it pretty useless as a cross-class.
How much dps should you be doing as a MNK on a single target in Alex 1 Savage?
I use Foresight instead. The only way I would take that skill is either if the cooldown was lowered to at most a minute, or if it could be used whenever. I still question the developers using it as a cross-class skill considering Misery's End and Assassinate cannot be cross-classed...
You use foresight on what exactly? Throughout all of 2.0 and all the 3.0 level 60 fights, there is only one fight where a monk would take unavoidable physical damage where mitigation would be useful(t13's earthshakers). Anything else you could be hit by is magical, which foresight doesn't work on, or avoidable.
Most relevant fights you can use Mercy Stroke anywhere from 1 - 5+ times, depending on the encounter. A1S, you should be able to get 3 minimum, A2S as often as it's off CD, A3S at least 3 times, who knows about A4S. Past raids you'd usually get 3 or more.
You're giving up 600+ potency by sacrificing it, and more than once it'll be the difference between an add enraging somehow or you killing it. Please switch to using Mercy Stroke, for the sake of your team mates.