Wonderously done.
Beautiful animation, if a little inconsistent.
The intro was strong. It managed to transcend its often childish medium and have more impact and grit than most hollywood movies in the genre have. The lineweight, good voice acting and well designed shots do all that's needed to establish the tone.
The middle is the weakest part. The character designs don't fit in at all with the rest of the animation, but I can understand there was a lot of very fluid animation that they had to be subjected to, so it is excusable. Aside from the goofy character designs and movement these battle shots feel very weak. The composition, poorly masked perspective tricks and at times lazy animation(like when the entire upper body of a character has no keyframes when moving, but his lower body is very active) destroy the intensity of the fight. Granted this isn't supposed to rival the MCU, but some more dust clouds, a bit of screen shake, some better use of light and colour when there are things happening both in the background and foreground would have been appreciated.
Where this otherwise sub-par section shines is the characterization done through actions alone. The silent heroes speak very loudly through their interactions with each other and their fight styles. I feel like I got to know them better during this short fight than I know some of my favorite comic-book characters after countless walls of exposition and movie tie-ins. Their relationship to one another and personalities are beautifully conveyed, top notch stuff, and despite their costumes being very basic(i understand, lots of frames), their unusual physiques and distinct abilities makes them immediately recognizable.
Just to get this out of the way, the other weak part is the music. The sound is strong and exciting, but very by the numbers and forgetable.
The final scene, after the fusion is a comeback in full force. It manages to ride the momentum of the intense, if poorly portrayed, fight scene, take the intensity from the intro's millitary assault and reinforce it with solid animation, good angles and nice composition.
Very satisfying end, which doesn't linger for too long.
All in all, a masterpiece of Kaijyu and Superhero animation, done on a small scale, but effectively enough that it's tone and pacing rivals even the most expertly executed large budget ones. All of its weaknesses can be forgiven due to this project being so complex.
By the by, I see very little SFX, did you do this all in Flash, no After FX? If so, I can fully understand those weaker moments of composition.
5/5 Keep making awesome stuff, please!