About the lyrics of...
 

I don't understand a particular line from kiss o' kill/Ladies Night. Can someone explain it for me?

The gist of the line is: "Cry like we're laughing, laugh like we're crying"

Does she mean this in a literal manner? Or is that line a figurative expression that means something else?

I think she means that we laugh so hard that we're crying and we can cry as easy as we laugh. It's not a literal thing.

She means that we shouldn't be afraid to show our emotions.

^Those lyrics are quite vague, but I agree.
Maybe ayu wanted to express that both feelings, happiness and sadness, are equal? Or more precisely: No matter which feeling, we can show/share it naturally?

I'm thinking along the same lines as Aga-chan's first thought. That both feelings of happiness and sadness are equal, in that, we feel both strongly or may react what's opposite of what we expect. Maybe even, not taking the feeling of sadness seriously, like laughing and getting over the negative and putting up a front of happiness, laughing but it's not a true "laugh" so it comes of as emptiness?

i guess the meaning of the lyrics can only be made known by ayu herself.