Make sure you've freed all 4 Dark Warriors. If you have 14+ Shurikens, that's good, but you can also defeat Dark Cloud without Shurikens. On the first turn, have Sages cast Haste on your Ninjas while the Ninjas attack. On the next turns, have both Sages cast Cure4 on everyone, and hopefully you'll kill Dark Cloud before her Dark Wave finishes off your party. There's also an alternative healing strategy that I use myself, although it's a bit risky. It goes like this:

Turn 2:
- Sage1 casts Cure4 on Ninja1
- Sage2 casts Cure4 on Ninja2

Turn 3:
- Sage1 casts Cure4 on Sage1
- Sage2 casts Cure4 on Sage2

Turn 4, 6, 8 etc:
- See Turn 2

Turn 5, 7, 9 etc:
- See Turn 3

If you're lucky, your Sages are able to cast Cure4 before Dark Cloud casts Dark Wave, so your characters will have to survive at most 2 Dark Waves on a row.