You don't want to unleash Archer's Noble Phantasm so early - not when there may be another way.
Lead Berserker away from here. I'll take care of Saikou, and then he won't be a problem. You're surprised at how calm and focused your thought-voice feels, and you detect Archer's surprise on the other side. He laughs as Berserker leaps into the tree next to him and he pulls out his machete to slash at the creature's claws with his left hand, then leaps backwards, landing another dart.
Alright, Warchief. Show this shaman what you're capable of.
You look to your allies - Caster is dragging LeBlanc back the way you came, out of the field of fire. If he could be freed to fight, he could likely crush Saikou, but it looks like he's still dealing with whatever curse is on those bone spears.
You contemplate hiding, trying to buy time for Caster to finish, but you discard the thought. You don't know how long it will take. Besides, the darkness seems to be Saikou's place, not yours.
You stride forward into the golden light of the Key in the clearing, athame held in your left hand, the red crystal of your Amplifier Code glowing. "The Age of Gods is over. The world belongs to humans and mages now, and I'm not letting you give it back."
She smiles widely. "You must be Naja Mishra, yes? Those of my followers who know of the Association said you were someone to watch. Let's test that." She levels the staff at you, speaking an incantation in what you think is Japanese. Swirls of dark green mana form around it in the shape of serpents that fire forward at you - some kind of curse.
With your right hand you form a barrier and with your left strike with the athame, infusing it with mana as your slash at those serpents you can't block as you move to the right, keeping her in your view. Once her spell ends you raise your right hand, your family's Crest channeling "Pyros!" as you fire a stream of white-hot fire at her like a blowtorch.
She speaks again, chanting quickly, and a barrier of spinning air attempts to disperse the blast, but some makes it through and she reels back with a burnt shoulder. Your assessment might be right - you have an edge in Magecraft. Your family's spells have been developed in the cutthroat competition of the Watchtower, while you guess she didn't have the benefit of such a formal education.
She calls up a swarm of other curses, bedeviling things like imps that fly and sting you, but while they dart in and bite at you, but you focus through the pain as blood runs down your skin. You throw the athame at her, and when she tries to shield again, this time raising earth, she hisses with frustration when the mana imbued into the knife lets you move it to slice through the shield's enchantment.
You try not to think about Archer as you hear the faint sounds of battle elsewhere in the forest. As this struggle with Saikou eats into your mana you can only be grateful for his Independent Action. Berserker must be a strain on Saikou's resources, another help to you here. Archer's blocking his link with you, trying not to distract you, but you can still feel him take a hit and you firm your resolve.
Saikou's blocking with mana infused shields. For a magus trained by nature-spirits, that's probably an instinctive way of fighting. She no doubt has ways to handle curses as well, but what about an attack she can't block with a shield? There's something you've been working on, not traditional at all, but you shout out the spell, as quickly as you can. Even as she shouts another command and the swarming curses double in size and rip at your flesh again with renewed vigor, blood running down your clothes, you chant:
"The Sun gives life,
Our hearts beat with borrowed flame,
Blessed be the
salamander,
Thief of fire,
Let night and winter steal away breath."
Mana leaves your hands in a wind and a circle forms around Saikou, and you feel the spell latch into effect as the heat in the air around her and within her body is forcefully pulled away. She calls up another shield, but there's nothing to block - the spell is pulling away from her, not attacking her directly. You see her skin whiten and start to go blue, and realize with a sick pain that you're going to kill someone. You could offer surrender, but you know she won't accept, and stopping channeling the spell will only give her time to recover, maybe call up something truly terrible. Archer, LeBlanc and Caster are all counting on you.
Just as you see her start to shudder and jerk in what you hope are the final throes of death, a warm, masculine voice says, not unkindly, "I'm afraid I can't let you kill her." You feel something strike you several times in the back and you go numb from torso down, falling down onto your hands, your concentration on the spell breaking as you land on your side, giving you a view of the interloper.
The mystery Master from before, choosing the worst possible moment to arrive, his Lancer in tow. He stands over you while Lancer goes to check on Saikou while you watch helplessly. The Servant looks up and shakes his head. "Another few moments and she'd be dead. She still might. She needs healing."
You look up at the Master and try to appeal to him. "She.. doesn't deserve it. She's insane."
He shakes his head. "She probably doesn't. But it's part of who Lancer is."
"Stop him. Use a Command Seal." You're desperate, there's no reason for him to listen, but you try.
"I'm not ready to kill a helpless woman tonight by leaving her to die. I hope the War doesn't come to that." He goes to the Key and takes it off the log.
Before he goes, you have time to make one more plea to him.
[ ] "My Servant… he's fighting for his life against Berserker out there. Help him. I'll owe you."
[ ] "My friend… another Master. He's nearby. She hurt him, badly. Caster was trying to heal him… maybe Lancer can help."