@Charus Only advice I can really give you is make sure you have a shifta drink, miracle rations, stirfry, and alliance 20% attack buff. I finally S ranked her today using the above with a +35 ice element fornis katana with crappy affixes (and a +35 fornis bow at the last moment for some desperate final nemesis shots) and the default store bought union set for units. It took me hours to finally get under 3 minutes, but I could relatively consistenly clear her with A ranks otherwise when I didn't completely derp and eat one of her attacks. Also got hit by the very first attack an embarrassing number of times lol.
Also try to stay in her face with the katana since countering will hit twice if she's in range of your normal attacks. I think my counters hit for 60k+ each hit with gear release and still in the 50ks without during the phirst phase, and in the 30ks with a shifta essence in the second phase.