hmm..it never seemed to bother me whether it became hard or easy on those routes..
but since you gave the advices...then i just add on a few..
Ranpha were mean to take down those fast fighters
while Mint and Forte were actually tankers..so use them to take down those battleships..Elsior fair against battleships too
Vanilla actually deals as much dmg as Milfie..so use her frequently since her special skill is AOE healing
and Milfie..being the best heroine..she deserve to be on your front line at all times..
just use her to break down enemies formation or use her as decoy/rescue miss