Why is it so good.

1 000 000% better than manga.

I am glad I am alive to witness this episode.

Cowardly or not, the only tactic that matters is the one that ends the battle with you alive and your enemy not! Concealing your mana seems like an incredibly useful deception tool that I'm surprised isn't more common in the universe. Then again, it seems like a tactic exclusively meant to deal with demons, and they don't seem like that common a threat (on the other hand, there is apparently an entire branch of study dedicated to reverse-engineering and improving demon made spells like Zoltraak, so who knows.)