This game is extremely hard on Hard difficulty. Take the game slowly, learn the game's more advanced mechanics, and analyse missions so you can develop strategies that will work on that mission. Complete the game on Easy first, then move on to Hard. The difficulty achievements do not stack.
Thanks to Jackanape for the following tips:
- When you take over a control node, be sure to consolidate it before you move on - so jump to it and build up the defenses, etc. to the maximum level. Also, leave a few units to help guard it too.
- Always group your units together. You can only put three in a team but it helps a lot. To do this highlight a unit and press LB, then it will show all the units you can join with so move to them (while still holding LB) and press A. Your guys will them be grouped which gives you better control over them and concentrated firepower.
- Be aware that you won't be able to see your guys shoot while they're moving (as they are dumb) so always move them in front of enemies rather than clicking on the enemies themselves. This way they will start attacking much sooner.
- Best guys early on are the infiltrators (aka snipers) as they have super long range and can often kill foes without them even shooting back if you get the positioning right.
Refer to THIS THREAD for video walkthroughs of every mission.