No it wasn't bad. I'm not one for film editing but I'll give my opinion. While I found the trailer satisfying some of the effects where a bit too much, and made it campy. Granted some of the effects were also pretty good for what you were working with.
I also enjoy your choice of Shepard (hes pretty cute, but that's just my opinion). However he lacked the urgency of what was going on. If I saw a bomb next to me I would probably not stop to deliver a line, even if I am a badass such as Shepard. Also: Is Mandalorian a Merc? idk. No real development on that part.
Best thing by far is the costumes, Loved the design, the atmosphere could also be engaging if used right.
If you don't mind tips from a amateur I would give a few tips on Shepards movements and actions. When the bomb comes close I would just have him run away maybe direct him to have a more stressed look. And when the bomb goes off its unlikely that he would come out with his head face planted down with his arms and fingers out like that, If he was knocked out I would have have him on his side, angle the camera in front of his face and have him have a pained look as he came to. If he wasn't knocked out I would keep the positioning of the camera where it is but be sure to have him defending his head with his arms and hands, its one of the first things they teach you in the military.
Also when Mandalorian goes down in flames he lands kinda awkwardly as well, I would use the terrain to show that he actually crashed, scuff him up a bit even if its just dirt. Shepard should also have more projection when throwing the bomb back. Maybe have Mandalorian crawling for it in pain and not reaching it in time having it blow up in his face. He just looks kinda like a turtle on its back trying to grab for it.
One more thing that I saw but not really all that big: I don't know if a plasma/biotics gun would have a recoil. Having it more stationary would give a more badass allusion.
These are just my opinions and I'm in no way trying to critique it in bad light. This is a lot better than I could ever do.