Like the title :) Somehow the ML who's "weak" always ends up in trouble and the FL who's "strong" always ends up saving him.

don't know fit your criteria or not. But maybe you could check

Oh My General. At first ML kinda underestimate by everyone, while the FL is the general. ML at first sometimes get into trouble & FL help him. 

My Mr Mermaid

She and Her Perfect Husband -  sorta fits. FL takes the role of protector and knight-in-shining-amour so to speak.