So, sneak in with a fire arrow...

And boom.

First they need to locate the Blaze, though.

Means they'll have to sneak in somehow. Easier for one or two people to do than the whole warband.

"We've worked well together so far."

"Volunteering to come with me?"

"Insisting."

"All right, then."

There's a wall visible from the tower they're gathering at that looks scalable... (To a Nora Brave; the killers probably couldn't manage - so might not realize it's a weak point.)

Gets them up under cover once they're over, too.

And onto the roof above several of their enemies, either ones keeping watch outside the Ring or standing and talking separately from the main group inside. 

Good to take them out, they should be moving quick enough that they won't be missed.

And with the two of them, they can take out anyone having a conversation simultaneously - give them no chance to scream. 

Exactly.

Gets them a clear path to an overlook, soon enough. 

Aloy will use her Focus to try to pick out the Blaze storage.

Seems to be on this half of the oval - but closer to the other wall, enough it might blow a big hole when it goes off. It doesn't look like there's an intact walkway over there, though, so she'd have to sneak past a lot of enemies to get to it. 

All right.

"Vala, go back and tell Sona to be ready to charge in through the wall on that side."

"You're planning to cross that alone?"

"Thick grass by the pond'll get me most of the way, if I time the Bellowback patrol. Which I can do. Then I just need an angle on that shed over there," she points, "and they'll have bigger problems to worry about. Don't need to cross the whole distance, I can lob a bomb from the hillside."

"...Alright. I'll be the first one through to back you up, though."

"I'll be fine. Just make sure to hurry."

"As quickly as possible."

"Yep."

And she'll head off, quickly but quietly.