BOSS ANALYSIS : Not much that will be unexpected here. Dark Shiva’s physical Attack seems to have a Dispel effect attached to it that will even get rid of Auto-Life, so that’s not nice. There’s also Heavenly Strike, the biggest danger of the bunch: it will inflict KO, Berserk, and Confuse in addition to some damage; obviously, if you don’t have Ribbon/Deathproof, you’ll only get KO’ed. Finally, Diamond Dust is non-elemental and hits everyone; for once, it has nothing special attached to it, other than being unhalvable, but still very predictable.
MAIN STRATEGY : You will pretty much NEED Ribbon for this battle, or a combination of Death-, Berserk-, and Confuseproof; otherwise, Dark Shiva will likely just keeping bashing ice against your head until you die. Auto-Protect is also invaluable as it halves the damage from both the basic attack (which won’t get rid of an Auto’d ability) and Heavenly Strike, putting them within a survivable range. And, of course, Auto-X-Potion will basically nullify that after with sufficient Defense such that you live.
If you don’t have Auto-Protect, then it’s wise to open the battle with Mighty G. Don’t bother with anything more in that regard, as Dark Shiva’s likely just to kick you to remove it. Even so, Auto-Haste will also be invaluable; it’s the only way to beat Shiva’s 255 Agility without having to put it back on every other turn. And, even then, you might not outspeed her - it seems her attacks have an advanced recovery rate like Quick Hit/Pockets does, so the higher your base Agility coming into this fight, the better … too low and she’ll get all of the turns she needs before you can open your mouth to complain about all that red in the CTB window.
… Wait, you got a turn? Great! Throw up Mighty G or Hyper Mighty G (whose utility is likely to be wasted over Mighty G’s, but whatever, it gives us a fighting chance), then go into an infinite series of Quick Hits. Hopefully, you’ll manage to get her down at least a significant amount before her next turn rolls around.
Basically, all this fight is about is avoiding Heavenly Strike’s ailments and maximizing your efficiency on the turns you happen to get. Just remember to send in cannon fodder Aeons before Diamond Dust, as usual. Once again, Overdrives can be used to the maximum.