Reapers of DXD (DXD x male reader)

Train your flesh so it won't burn in hell.

Time: The first weekend after school

Place: deep in the forest away from the town.

You decided to make the training a bit harder. Until this point you were giving them rather simple strengthening exercises but now you decided that they are ready to start pushing their magical capabilities. 

(Y/N): Now the girls we'll start with our black feather unit. 

Girls simply looked at you with a face that was saying "Really? That's how you call us?"

(Y/N): Girls this is literally the first time in history that a reaper recruited so many people at once so we don't really have any official names. Besides it sounds better the "former fallen angels". 

Now then As you all definitely know magic weapons attacks and all of that is constructed from magical energy. The strength, durability speed and the time said constructs last depends on how much energy you've put into it and how you've formed it. 

For the start we'll work on the lasting time of your constructs. Your job is to create the best spear you possibly can and keep it active. 

Meanwhile I will create a fog that will accelerate the dispelling time. With this you will be learning far quicker then normally. Once we'll deplete your magic Asia will use her sacred gear to recover your stamina. 

Rinse and repeat and your stamina will increase significantly.

Ok?

The girls nodded.

With that said girls created spears of light. Right now their weapons looked just like in anime but with different colors.

They focused their magic energy. At first they were feeling pretty smug since weapon creation was completely effortless. However as soon as you started accelerating time around them, their confidence melted away like ice in a microwave.

After lasting for around five minutes, which roughly translated to 2 hours they collapsed on the ground with no more strength in them.

Asia: Umm does that mean we are done with our training?

(Y/N): Nope far from it. This is the part where your abilities come to play.

I'll teach you how to restore magical energy and stamina. One of the most useful abilities for someone that is focusing on support.

You explained How she's supposed to circulate magic energy and how she should focus on the muscles and enhancing the respiratory and circulatory system since it's not only responsible for regaining both stamina and magical energy.

After that she was able to perform the technique quite easily. Girls still felt sore muscles but other then that their were fine. 

Asia: I did it!

( Y/N): Congratulations. The fact that they were able to recover so quickly despite the fact that it was the first time you used this technique is astonishing. Few more tries and you should be able to master this.

Now then Let's repeat all of it.-You said with a menacing voice.

The girls felt rightfully scared since the training lasted for a whole day. After 3 whole days of such training girls were able to hold their spears for 24 hours straight and Asia could bring all three of them to full strength 20 times.

(Y/N): OK I thing that your stamina will do for now. Let's move to other things.

Next part will be magic control. It'll allow you to use far more difficult techniques and on top of that you won't be losing magic energy so quickly.