The episode 6 idol song scene. Just two words: Future Star!
The end of episode 11. After acknowledging that he's no longer immortal, Akihito rushes through the city dodging Youmu obstacles to get to the highest point and jumps into the alternate dimension. But he doesn't just smash in, he rips through with his Youmu hand, having regained some of his powers and stay in control. For a guy who's had very little action throughout the show, this was quite an scene to watch.
Episode 12 features Akihito, Mirai and True Form!Ayaka in an all-out war against thousands upon thousands of youmu. They're tearing them to pieces left and right, particularly Akihito, believe it or not, who's exploiting the fact that he's gotten ahold of some of Beyond the Boundary's power to destroy youmu of all shapes and sizes with ease and by the dozen. And then he follows it by beating Beyond the Boundary into submission by making it understand that all of the negative feelings it embodies (as it takes Akihito's form) belong to him and no one else, before sucking it back into his body.