Always have a medic near your tank units, for it makes them nearly unstoppable.
Keep your snipers back - they die quickly, but can be devastating if you use them well.
Soldiers make very effective walls, with only the toughest units able to take them down in one hit.
Watch out for enemies hidden from your sight.
Any energy you don't use in a turn will carry over to the next one.
Try and keep a few units situated near your base so you can be prepared when/if the enemy draws near.
Pay close attention to the HP of other characters. If you approach one, attack, and they continue to live, there’s a chance they can kill your unit in the next round.
A good way to get a view of the other player’s actions is to send over a lowly runner character.