I'd put DG+ at A. 10% chance of taking no damage is amazing no matter which way you slice it. Not 100% necessary, but it's still really good.
Luna I would actually give a B, because similar to the also B-rated Breaker skills, it's not universally useful. There are a lot of situations where Luna only adds like 5-6 damage (fighting mages/archers/P.Knights for example), and even at extremely high skill, it's not better in those situations than getting a guaranteed +5 damage from the (again B-rated) X-Faire skills. It is still really good for tank-busting, and bonus damage is never a bad thing, but there are often other skills that can perform just as well, if not better than Luna.
Koitsu wa mirai wo takusu eigo no tsurugi da! ZANKUU TENSHOUKEN!!!!
Luna is a great skill and all, but like someone said, there are times where it isn't going to add all that much damage. It's great and amazing vs generals or other high defense units if using physical, and great vs falcon knights and mages for magic damage dealers. However, it's still got it's own limited uses.
Dual guard is personally my favorite defensive skill due to how useful pairing is and it blocks everything. All astra hits, lethality, pretty much anything.