This Hero Versus Derimus Imun!

My foe and I stood opposite of each other within the decimated room. The sun has already dipped below the horizon and the dark starlit sky has begun to emerge. The dust that had filled the room was now completely gone, having left through the many holes that had been created by the battle prior.

I focused my attention on my party behind me for a moment. I could feel their individual energy swirl within their bodies. I am unaware of when I obtained this power, but it feels like it's a more precise version of the ability to detect mana from my magic casting classes.

Everyone behind me is unnaturally weak. This fight has only been going on for about five minutes, but all of my party members are nearly at zero energy.

I understand that they have been fighting a Hell Guardian, but even in that case that is no excuse. Everyone in my party should be able to fight non-stop for at least two days straight if they maintain a reasonable level of energy usage.