If you're within range of the SENSO-Gate attack it. Not only you'll weakening your opponent but you'll also move away his troops from the map letting you easily dominate the most of it.
To make even more destruction try to try to save Combo Cells at the end of your turn of an easy level and use them at the very beginning.
The more units you have the slower progress of your opponent will be.
Keep in mind it's far better to have a mix of cards upgraded equality and in small numbers than having a large set of low level cards.
If you can use Chuck, Warp or Shuttle to bypass defenses for a surprise attacks on enemies SENSO-Gate.
Don't placing your units close to each other so you can avoid devastating effect of area damage cards.