Yuusha Party wo Tsuihousareta Hakuma Doushi, S Rank Boukensha ni Hirowareru ~Kono Hakuma Doushi ga Kikakugai Sugiru~ - Ch. 18.2

[flannan]

The only sensible reason I could see is the MC's side believing their chances of winning, despite their strategy, are still extremely slim or that they would at least suffer casualties before defeating the demon. So, explaining all that would be a sort of bluff to make the demon withdraw. If the demon's whole "strategy" was to rely on that magic armor, then simply knowing it's not working fully anymore might make the demon consider the battle not worth it anymore, as he could be risking his own life now. There could be some merit to this, as well, because the demon hasn't actually managed to kill anyone from the party yet.

But of course the real reason is the idiotic shounen trope of needing to talk endlessly with the opponent in the middle of a fight.

The other reason, is that the demon's power works actively. And so, if he starts panicking, it will be working worse, giving them ability to kill the demon with more ordinary attacks.
Generally, attacking an opponent's mental state is a valid strategy, often used by heroes and villains in manga.
Reminds me of that video game, where my party defeated Dracula by bullying.

 

[androgynous]

I said it before, you’re explaining your actions to your enemy only to trick/confuse him while actually using another attack!
All this expositions for the readers sake can be done later! This is just wasting, an already small amount of pages per chapter

 

[sephtis]

Main characters went to the Rose school of giving the enemy your plan.

 

[SilverSour]

You explain what happen after winning, not during the fight you stupid ass

 

[welcome2atlantis]

Yeah yeah, go ahead and explain to the enemies how you're going to defeat them. Reveal all your cards. Let them work out countermeasures when they meet you again next time. So kind, so big-hearted.

 

[XyRwins]

Why the hell are they explaining so much bro

 

[Coldlight]

Oh wow, blabbing the nitty gritty of your sneaky strategy to the enemy seems like a very smart idea...