Don't build too many of one unit type, this makes them easy to counter. The other night I faced two different people who built like 3 Grimbarts. While this was strong against my early infantry I was able to just kite them until I got some anti armor units to counter them.
The importance of counter
Learn unit counters and know when you can win an engagement. If you walk into a fight you can't win don't commit to it. Just get out of there. Push another location and build units to counter their army composition.
Defenses and Turteling
If they build defences and turtle circumvent them and hit their backline mines force them to move, demand their attention and cause damage elsewhere until you have units to break down the defence.
Always manually position your units to make them effective. Especially when a mob is capturing nodes. You will notice your units have a hard time killing a mob when capturing a nodes if you are using default attack. You need to make sure all the guns of your infantry are line up and their line of sight is not being block to make them effective.
Usefullness of meele
Engaging in a melee is not a bad thing especially you want to pull all mobs in a chokepoint then your flamethrower will burn them all. Also engaging melee with flamethrower and infantry machine will make those unit ineffective.
Repairing vs Reinforcing
Repairing a mech is always more cost efficient then reinforcing it in base. This means you should only reinforce your mechs if they are to valuble to not instantly heal to their full health.
Mechs backwards movement
You can move mechs backwards using the "C" key. Most mechs are slower when moving backwards, but you will not have expose their rear back.
Early game focus on capturing all of the resource points and flags on your side of the map. This should be your top priority. So many new players I play against seem to capture a couple and then get distracted and forget to capture the rest of them.
Micro your units
Keep your units busy. There is almost always something your units can be doing: Capturing a point, harrassing enemy units, scounting. If they are just sitting there then you are giving your opponent an advantage, also they are a prime target for a grenade throw.
Reatreating is a good tool
Retreating units is always more worth then gaining or holding ground. Losing a single unit can lose you the game, but losing the control about a mine or pump for a short time is not as bad.
Figthing to get room
Don't be afraid to be aggressive and meet your opponent, the further out on the map you fight them the more room you have to fall back on or move your troops. This is especially important in the early game.
Pick up weapon crates. On each map there are weapon crates that your squads can equip. Learn where these are and send engineers to pick them that way you get your troops quite cheap. The only one that is not worth picking up is the medic one. Its important to pick these up early so that your opponent doesn't take them.
Maximum amount of information
Enabling the first two of the shown options will help you too see if a unit gets show or if it hits something. The third one will help you to position your units at the best range possible and to keep out of the range of the enemy units. The last one is will increase the speed, because you do not need as long as when lowering the setting to see the information.
Track your progress
Enabling the time will give you the chance of tracking your progress between games. Ingame it helps to predict how far your opponent is and how long it will take him to get to the next milestone.