Ancient Knowledge when to use?

[Metathron]

I'm assuming you'll level up fast(er) due to the use of AK scrolls and so need the excess troops to "fill up the edges" anyway. The rest can wait in your reserves.

[BB Shockwave]

I have only found one AK scroll so far, they are quite scarce.

But I did use a Burning Eyes scroll to defeat the Elon map guardian in Verona, it was a tough battle (about 130 Knights were the biggest pain there) but I managed to do it no-loss.