(Or move around a lot, really really fast)
..annyway, here's the plans.
First up: There are two new entries on the skill list. "Dreams of Silicon" and "Dreams of Iron." Might wish to use those or not.
In general, I've got tactics for three different sorts of engagements.
These are tactics that can be used while remaining relatively human, moderately conventional things that could be done, and can maybe be mixed into higher level plans. This is called 'screw destiny', as none of these tactics will require Phoebe to become some higher form of life, and grant her more freedom of choice after the battle.
First off, we're going to need time. Time to recharge that shield, and time to maybe find more energy to take in. We're still mostly tanked up and can possibly take in more raw energy. Need our friends to buy us some time to recharge and get ready to fight again.
-Sir Isaac Newton. We have the ability to make high-speed trajectory calculations and the ability to fly seriously well. With this, we can take off again and deflect the largely physical projectiles upwards towards lower orbit. The goal is to cause them to all re-enter at about the same place and time, so we can fly there ourselves and aim them on their way back down towards the creature, taking advantage of some of its own power expended sending these upwards. Specifically, we mostly run and direct the shots after they leave the creature's willpower radius, and aim them to come back down in a large group that we can manipulate to direct them at the Plague Lord, this time with us on the good side of the potential energy equation. It should allow us to hit with a lot more power than we can normally wield, while doing so with significant efficiency.
-Directed evolution. We have an implant that transformed our arm into a living weapon. Phoebe is a lot stronger and more capable than she should be, but this has never been really pursued with dedication. Going further should be quite possible. We should allow ourselves to be faster and stronger, the goddess of war that we are supposed to be, by reforming our body on the fly for further optimizations. If the creature can't hit us, we will do much better, and there is no sign that we're even close to the limit of what we can do while still retaining a mostly human body. In the past, enough has been gained to exceed our sparring partners.. and this time, our new partner demands a lot more. Get to the next level.
-Fusion lasers. Directed energy beams hurt the plague lord, but there is a lot more that can be done with them. Powerful lasers can ignite compressed hydrogen packets into small fusion explosions, dramatically multiplying their effectiveness. Basically, we fire small packets of hydrogen, and then hit those with lasers, causing them to explode and increasing the effectiveness significantly.
-Ion lasers. With a little extra electrical kick, we can also make said lasers (or other projectiles) create ionized paths below them. With a bit of work, we should be able to generate an electrical imbalance in the clouds, particularly since there's a storm going on right now. There are three possible abilities that can be used here.. calling down bolts of lightning behind physical projectiles that we shoot down from the heavens, having lightning bolts follow our lasers, or creating large cycles of current that we then disrupt with our other attacks.. causing electromagnetic pulses. The last is probably the hardest.
As for defensive techniques.
..we really should have some kind of physical shield. The Plague Lord uses them as mobile forms and propulsion methods. We can do so as well, bringing chunks of debris with us that we can use to deflect attacks, saving our actual shields and giving us other options than to just tank the attacks with our body. They can also be used as propulsion methods in a different way, giving us things to push off of to gain more momentum more quickly, as we have a higher-density bit of material around than just air.
Second, lets get us some manipulators of our own, some tentacles or whips of flame, to gather acid and give us longer arms than our own. Our wings are mighty in and of themselves, who is to say we can't have two more, four more, six more? The better to cover ourselves with, and give us some actual tools to help block or deflect the plague's projectiles. Focused manipulators will work better than just raw willpower.
That's A. some of plan A can be used with the others.
(I'm less sure about B, as I don't know the lore details, not by half. But Phoebe has been off-planet, and has access to some kind of spaceship to do it.)
You know that spaceship we have access to? Eat it. All of it. Compress as much of it as we can into that willpower radius, and either turn it into super-dense metal or coopt the systems into being ran directly by us. Any space travel method requires immense quantities of energy.. energy we can use. Doing this will be crossing a threshhold, and will require our allies to buy us a notable amount of time. It will also, if even partially successful, give us an immense amount of energy which we can use to utterly devastate that plague lord. If there are other useful systems, such as weapons systems, those can be applied as well. Even if we're just tossing super-dense rods from extreme altitude like multi-ton railgun projectiles, it will do a nasty amount of damage to the target.
I also suspect that deliberately grabbing thousands of tons of metal and attempting to make it part of the self will be an experience that will have permanent consequences.
The very first power manifested was fire, and the production of heat. The fact that our nanites are unaffected by laser beams, and treat immense temperatures like the sauna means we can take a lot more heat than the Plague Lord.
..so, we use that. Instead of spreading our willpower over a massive range, we bring it inwards. Take the tens of thousands of cubic meters of power, and compress it, as much as we possibly can. We don't necessarily make it a proper fusion reaction, settling for ending up at hundreds of thousands of degrees hot instead of the tens of millions, but we make ourselves thoroughly, immensely, hot, and simply fight the plague lord in melee combat. When this hot, the creature's nanites probably cannot infect us, particularly with our willpower compressed to such a tight radius. We're also likely quite a bit too hot to handle, and will almost certainly be damaging it through proximity, not to mention blows that will have literal tons of force from the highly compressed matter that came along with our manipulations.. the air if nothing else.
Also, if we're cladding ourselves with that much raw material, there's no reason to stick to a human or humanish form. Lets go for something more killy for the melee combat with a wannabe world eater. Or a phoenix if we want to use more conventional attacks from the air along with it.
Shoot, it might even be enough to tear away some of the creatures nanites again, this time with better odds as ours will be facing dying dust that is outnumbered massively, as the creature is both huge and subject to wild shifts in leadership.. It might not be properly capable of matching a one-will concentration strategy that plan C will require.
On the other hand, the experience of being something very analogous to an energy being will probably shape Phoebe's outlook forever.