I wasn't expecting much from the episode when it started, but as the episode progressed (besides the two main songs), the filler content was actually very enjoyable.
Criticism on the main songs:
Song 1 is kind of like that Soap Opera rap style (like Trapped in the Closet) but the execution I felt was a little weak for usual FOTC stuff. Song 2 (the Duel song titled from the episode title) was kind of like an Andy Samberg Digital Short. That wouldn't really fit with a FOTC storyline. (unless you're realy into those Digital Shorts)
But I'd have to say, the filler content surprised me as the episode progressed, especially the strobe lights remix and the final performance in general.
Except for the main songs, this was a pretty awesome episode.