you need to beat up grunts and gain morale to do stronger attacks, to henshin you need to ride his bike(although later on, there's no need for that), its basic SRW knowledge really, what's punishing in this game is you will be forced to use the units you have in and ONLY THEM, in certain stages, which has a possibility of locking you forever in that stage if you're not patient or lucky enough.
you can't spread out too much in battles, especially if you're a cheap guy who doesn't upgrade characters, only focus on 3-4 members at max and you should be able to kill all the grunts, have enough morale to transform and attack, and thus beat the stage's boss.