You probably want to note that Giga Flare completely ignores DEF and MND, so MYS affinity is the only way to resist it. Hell's Tokamak, however, has absolutely terrible piercing, so it is rather easy to have your tanks not take very much damage from it as long as you just keep your defensive buffs up.
Oh, and you'd better hope that she doesn't decide to spam Uncontainable Nuclear Reaction, since it buffs her MAG with 35% each time she uses it. If it doesn't kill you outright, the buff Giga Flare gets from it most likely will.
when her health is dropped down to 5,500,000 she will use Hell's Tokamak and Giga Flare
Not quite; Hell's Tokamak is unlocked at 7,000,000 HP, and Giga Flare at 4,000,000.