Depending how many DK's you have...

Soul Reaper looks to have a six second cooldown.
Meanwhile the dispel, which removes all effects (or is it magical effects), is usable every 8 seconds.
You're not going to be able to dump all of them on a target, and have it stay there for 5 seconds in arena.
Against a single healer team, you could put two of them and then another two after the dispel.

In battlegrounds or PvE, most of the time you'll have it up the necessary 5 seconds.
You still need to have the target at/below 35% for the bonus damage.
You could try moving it to a later point in your priority/castsequence/step macro.
That way the target is softened, the Soul Reaper is placed, and within 5 seconds of doing that the target is below 35%.

It's basically a finishing move; you can either place it where you figure the target will be below 35% five seconds later.
Or use it on its own keybind as a finisher... manually use it, rather than include it as part of your spam sequence.