Episode 10 of the second season fully establishes Sawa-chan as a Retired Badass.
The first time My love is a Stapler is played on stage. Yui forgets to sing. Mio jumps in to take the words. For the shy girl she is, thats pretty awesome.
Then they turn it into a pretty epic duet.
The seiyuu actually playing their instruments for My Love Is A Stapler and Fuwa Fuwa Time in the Let's Go concert.
And in the Come With Me concert, Pure Pure Heart and U&I (as well as performing Fuwa Fuwa Time again).