You have some great sounds, great runs that amplify the epic feeling.
Please excuse me, I'm tired and kind of thought "well I have some time to review another piece before I go to sleep",
so I'm simply going to lay out the points that are problematic from my point of view,
things that you might want to take another gaze at:
1. You might want to avoid the repetitive bass drum at the beginning.
I don't remember where, but I read somewhere that epic pieces should use big booms once in a while.
2. the part where the guitar gets in feels suddenly unconnected.
The easiest way, welcome in this genre, is to use a reversed splash cymbal or a "woosh to hit" to ease the transformation.
3. I believe that you use a little too many instruments for a 3 minute piece.
I tended to do it as well at my first two years when I "just wanted to make the piece go forward by adding a new color, therefore adding a new instrument".
This one leads us to point four.
4. Use dynamics more. Remember- you have to start small, or at least have some softer parts in the middle. You can grow back from there,
it's important that you let the listener's ears rest a little, and then give them something much greater in comparison.
I hears that around 2:10 you took out everything but the guitar but...
I don't know, it kind of felt like you decided to leave one color in your painting rather than painting in some warmer/softer colors here and there.
5. Some parts lack bass. Do you know the Inception brass? This "BRaaaah"? They tend to put these in epic music. (https://youtu.be/830I9w7I7wM)
I heard that you used piano bass notes, which is a sound that I personally really like, but at some parts it lost some of it's power because there just wasn't enough bass.
At least it felt that way for me.
Here's an epic piece I enjoy listening to: https://youtu.be/J-0TcwJT4go
Here she uses a group of small percussion instruments to "keep it moving" and makes good transition and scale changes using dynamics and harmonic changes.
Yes, she uses a lot of instruments, but the progress doesn't come from there.
I encourage you to take a listen to this, maybe to other boss fights to computer games?
Try to learn from there.
It's not that you piece isn't good, but you can always try and make it better,
or make the next one better.
Hope you have fun and that I wasn't too harsh on you.
I'm going to sleep now, hope that was helpful.