I find grouping up works the best. You can also place the 5th piece (pinky) behind slower units to push you bigger units into battle quicker.
Putting more troops on less would be the general strategy, but obviously, it's 5 on 5 so pick your fights. I have gotten at least two stars on most of our levels (didn't keep track) so I know they are possible. I wish we had more descript models for telling more about what's going on but we had some hiccups in animation and cut some edges short on the game, practice makes perfect.
Thanks for trying it, it helps all of us out.