To begin with, I liked the pace you took; if you had gone any faster, then it would have been more difficult to notice the different methods of attack, even if it would have had more tension. The interesting thing about this film, is that it is centered around a battle, yet the outcome is not the complete focus. What I noticed just as much, was that you were showing the various ways basic bricks can be used. From catapults, to castles, to cannons, the two fighters used different ways of combining these to attack each other.
Not to say your animation in other films is that bad, but the animation in this is much better in comparison; any mistakes are very minor, and not as noticeable as they are in your other films. Your walk and run cycles have greatly improved, and the somersaults were quick and smooth.
The ending was definitely a surprise;
Spoiler (click to read)
both of them turning into a dog for the green minifig. So neither one wins in the end!
The minimal but tastefully added affects were avant-garde because they were not anything really complicated; just some camera shaking and masking, but that was all that was really needed.
Some of the comments so far point out that it does actually apply to the avant-garde theme; such as creativity and originality (the only other brick battle I saw on youtube was just two minifigs fist-fighting, and throwing bricks already in existence at each other). Also, as you pointed out, the different techniques used aren't really in many of your other films. In addition though, you have improved greatly in many aspects of film-making.
Whether they are perfectly executed or not; many aspects of this film are avant-garde. The problems, however many, are well overcome by the relative improvement and originality in this film. Even if you aren't successful in the contest, your studio has surpassed itself in many aspects. Well done BuilderBrothers. 5/5
(LegoPinions: I don't claim to be better, but I beat your review in length ).