Select a level that you can earn a medal easily. Play the level repeatedly until you get gold. For example, to get an easy "Preserver Of The Infantry Medal", play the first level and win the battle without losing any infantry.
More units in Campaign mode
When you do a level in campaign mode, surrender, then play it again. Surrender again. When you play it again you will have all reinforcements ready. This works in all levels in Campaign mode.
Stuck highlighted
When you are done giving attack orders, there is a button with a "faster" function. Select it, and it will become blank while the CPU movement will increase. Tap the blank again, and while you are attacking the button will remain highlighted.