Careful tactics should be used to be successful in combat. Keep these things in mind:
Use crouching and prone stances wherever possible.
Spread out so that your team won't get blasted to pieces by a single errant grenade. Don't spread out so much that your mercs become vulnerable individually.
Keep your mercs aligned within view of each other to maximize combat interrupts.
Wear suitable vision gear for day and night conditions.
Stay out of lamp-light; stick to the shadows, as well as near walls.
Try to create an intersecting crossfire so that soldiers running in one direction are interrupted from the other. This is called an enfilading field of fire.
Avoid being exposed around corners.
In JA2 1.13, use pistols for shorter ranges where longer weapons become ineffective.
Fire from the rear. Try and ensure your guys in front remain undetected.
