in hard campaign you are have an option of which characters to choose unlike normal campaign.I highly sugget using rigged co's like sami-force march,kindle pretty good,hachi for cheap deployment,grim for offensive power,max melee offensive and jess for tank power.
I should recommend use at least hachi in your team because he has good stats with no drawbacks and his co power gauge is really easy to refill which is really good for tag mode for 2 troops movements in one turn.
If you have problem with several stages trying gaining experience in skirmish mode.Use skills that are useful in almost every senerio like extra damage on melee offense,decrease melee dmage in defense,100 gold per station and reduce unit cost