If you can't hone Magic Breakdown to R2 (entirely reasonable, 4* orbs aren't easy to come by), bring a honed Magic Break as well. Not as good, but any mitigation is better than none. I'd recommend rummaging the FFRK subreddit and its friend threads for SG peeps you can follow. I followed a good three or four who followed me back, so I'm pretty well set in that regard, but Sentinel's Grimoire is the best RW for those high-end boss battles bar none, 60% reduction to all non-Gravity damage that stacks with Protect and Shell is crazy good.
Tangentially, I've been peeking at the next batch of character events and man, they're padding out a bunch of currently non-existant skillsets. The Tifa reissue next week will release no less than -three- Monk skills (Pressure Point, Chakra and Aura Blast), two of which actually sound pretty decent (it'll also release Shellga for WM, which is great for all the obvious reasons). Eiko event will be a hueg gift basket, releasing two new Bard skills, two Knight skills (including Taunt, marking the rise of Loserface's Tauntaliate age or something) and even giving us a second shot at nabbing a free Phoenix, along with a couple lower-level summons like Shiva and Bomb if you happen to care. The influx of underdevelopped skillset padding will die down a bit past that, but there's still stuff like Diaga and Faith to look forward to in Aerith's reissue, for an example.