Awakening hard mode tips?

For context I'm still new to the series, I've only finished Echoes and Shadow Dragon ds.

Everything was going smooth until the chapter 5 fight against Lucina, my units felt massively underleveled. Tried the paralogue and it was the same deal,

Also same story with even the lowest bonus squad (did the trabador one who has 17 hp and my units are hitting for like 3) ///////

Is it better for me to focus exp on just 4-6 units instead of evenly spreading it amongst everyone?

Also I feel like I'm not using the pair mechanic effectively

Thanks