Don't Use This:
Greninja @ Life Orb
Ability: Protean
EVs: 252 SAtk / 252 Spd / 4 Def
Nature Timid
- Hydro Pump
- Extrasensory
- Ice Beam
- Dark Pulse
Why It's Bad:
Dark Pulse Greninja was relevant when Aegislash was dominating the meta, but with its recent banning Dark Pulse is redundant coverage these days. The main things you'll be hitting with Dark Pulse are already handled by your main Water/Psychic/Ice coverage and the couple of Mons that aren't tend to not be all that common anyway.
Instead Use:
Greninja @ Life Orb
Ability: Protean
EVs: 252 SAtk / 252 Spd / 4 Def
Nature Timid
- Hydro Pump
- Extrasensory
- Ice Beam
- Grass Knot/ Hidden Power Fire
Why it's better:
Grass Knot lets you hit hard and finish off many of the bulkiest of Water types, either 1 or 2HKOing the likes of Starmie, Blastoise, Slowbro, Politoed, and Quagsire. It also allows you to nail most Tyranitar for around 70-80%, giving them a hard time switching in and picking them off without the risk of Hydro Pump missing.
Hidden Power Fire on the other hand. allows you to take down Ferrothorn and Bulky Mega Scizor with relative ease, being another nice alternative with good coverage.
Conclusion:
Unless you're afraid of Espeon or the occasional Alakazam, there's no reason to be using Dark Pulse over another coverage move. Grass Knot and HP Fire hit a much larger and more relevant range of threats, and Dark Pulse should just be hidden away until the eventual attempt to re-test Aegislash one day in the future.