SP grinding dragons is suffering

Good advice all around. A damage special like Moonbow would make things much easier. I imagine that's what you want to give your Ninian in the long run anyway. Boosting Ninian's Spd to double could help too. If you plan to eventually give her Fury 3, you can use Fury 1 or 2 to get her Spd to the necessary tier, along with a seal or allied boost. You probably don't want to use Fury 3, since it's too much chip damage.

If she takes too much damage from the greens and you really don't want to have someone else deal with them, take someone along with Hone/Spur Def 3.

If you have a low damage healer with Pain, like Azuma, that can go a long way too. He's nice since he can easily tank, heal, and chip.

A final suggestion is if you find your Ninian having difficulty even killing reds, you could consider giving her TA1 (assuming you don't plan to go TA3 in the future, if you do then focus on TA2 ASAP). It's an extra skill and may seem like a waste of SP but it will make her easier to grind in the future. In that case, I would really take a second unit along to train them on the non-reds or at least the greens. This last suggestion applies to any unit with very low Atk. Even if you don't plan to give them TA3, sometimes the small waste of SP on TA1 can make grinding them so much easier, especially if you know you have a lot of grinding for them. The nice thing is Selena can give them Reposition too, though that obviously doesn't help dancers.