I just finished the game and thought I would pass on a few tips... Some of the later levels are a little tricky. Warning! This guide contains spoilers.
Tactic Tips and Tricks
Rifle Team (LAW)
These guys are the backbone of many maps. They are absolutely fantastic at taking down enemy armour with their light AT weapons. They have a camouflage ability which means that units must be adjacent to them to engage (with the exception of artillery).
Consider placing them supporting each other and on either side of your dedicated AT infantry.
Anti-Armour Team (Dragon ATGM)
Another backbone with defensive gameplay in mind. They have a range of three hexes and carry 2 AT missiles. Unfortunately they do require a turn to set up so ensure you leave time to do that. I will typically support them with at least one infantry team.
I have lost many teams to a quick rush from enemy armour and with their two health bars they will very quickly perish. They are a massive magnet for enemy artillery so consider placing them in an armoured vehicle for cover if they are being held in reserve.
Consider placing a truck or jeep with ammo to resupply their limited ammunition.
This is a handy team that is best used like a scalpel. They have a 2 hex range with their M21 rifles and can cause 1-2 damage which makes them good at helping pick off enemy infantry. However they have one lease health than your regular infantry team so it's best to keep them in support.
Their main feature is the ability to call in an airstrike which will make short work of anything they can target. Unfortunately the range is a woefully short three hexes which does limit this ability somewhat.
It is important to look at the small window in the top left of the screen which indicates the availability of your support. The turn after you use the radio is the turn in which you will be able to designate the target.
A useful trick is to have your Recon team one hex back from the front line in cover and to use the radio. The next turn, move your unit forward and designate your target (this can all be done in one turn!).
The old faithful APC. I wont go in to all the variants but some have a mounted HMG and some carry AT missiles. Relatively delicate but useful when supported.
Your attack helicopter - great at almost everything but may as well be made out of spun glass in terms of durability.
These are very expensive but provide excellent fire support. Great for locking down an area but watch your corners. A quick rush by enemy AA will have you cursing.
Consider using them for priority targets and denying capture points.
A workhorse with moderate durability. Best used in pairs to rush targets, watch out for those enemy AT missiles. Remember to dismount your infantry before rushing in!
Awesome tank but not cheap. Great durability especially when paired with smoke. Make use of the move and shoot. Pairs well with another Abrams or a Bradley in a pinch.
Good all-round vehicle. Comes with mounted infantry which makes this very useful. Tow missiles can make relatively short work of the enemy but can be prone to getting rushed and one shotted. The cannon has good range and chews up enemy infantry and helicopters.
I like to push my infantry out as a screen on some maps. This way they'll be able to get revenge on any tank that gets a lucky shot.
My usual go-to for shooting down helicopters and cutting down enemy infantry. Cheap, with a little bit of armour and reasonable mobility. The range on the gun is good and it makes a good pairing with a Bradley in a defensive position.
About as much use a chocolate teapot when it comes to taking on enemy armour of almost any type.
I want to like this but it's one of my least favourite units. This is an AA missile launcher with awesome range. Unfortunately it has no mobility as it requires setup and the lack of armour means it usually gets destroyed by artillery quite quickly.
Light observation helicopter with no weapons. On some maps it is very useful - particularly as a spotter for artillery or to let you see what's headed your way. Nice and cheap too!
Standard transport helicopter. Great range and mobility - I like using these. Comes with infantry, usually worth buying a few. I believe the recent update allows these to fire while airborne now which makes sense.
The heaviest of your artillery options. Not that this means anything in terms of damage but it does have the range to hit any hex on the map. Also has one armour which makes it essentially immune to artillery fire. Does require a turn to set up before firing.
Prioritise enemy artillery if possible
Your well constructed defenses will wither away under a constant barrage from the far side of the map. Consider an airborne assault if possible - either an attack helicopter or quick troop drop to assault the enemy gun.
Be aware that the mobile artillery isn't defenseless and it will take pot-shots at your delicate helicopter.
Another option is getting a spotter in place and hitting it with your own artillery if the target is unarmoured.
Prioritise enemy helicopters
Some of the maps are brutal with not providing any dedicated form of anti-aircraft. The helicopters will whittle you down while you're unable to doing anything about it.
Jeeps with a heavy machine gun (HMG) will be your best bet here. They are cheap, have a large movement range and if you swarm a helicopter you should be able to take it down quickly.
Don't forget! The crew can dismount their machine gun and set it up which can be useful for locking an area down. APCs and Bradleys are also handy AA assets in a pinch.
The AI cheats like crazy
Unfortunate but true. Before moving or setting up your defenses - look at the ranges and overlaps of your units. The AI knows where your units are placed and exactly where the holes are. There is no fog of war for them...
Box those tanks in
The enemy really doesn't like AT infantry or TOW missiles. Use that to your advantage! In some maps you might be able to pen them in an area which leaves you free to capture or destroy to your hearts content.
Mobility is your friend
Particularly with helicopters. Use that massive movement range to drop troops at choke-points or other areas of importance. You also don't need to drop ALL of your infantry. A couple of rifle teams in the right area can take out a couple of tanks and buy you valuable time.
Trucks are fast
It might be a little cheap but it works. I've used the huge movement range of trucks (especially when on roads) in order to capture points and rush the objectives. The great thing is that they're usually not very high on the priority list for the enemy to target.
Capture the radio
I've seen a few people wondering about the radio - some maps will have a 2 man radio crew or jeep with the radio in their loadout/equipment. Simply destroy this unit and complete the map and it will be considered captured.
Armor is useful
The little minus signs in the health box of the unit denotes their armour. This regenerates and shows the toughness of the unit. Any unit with armour is immune to artillery so save it for the enemy infantry.
You will need to overcome this armour to destroy the unit which may take multiple shots (may the RNG Gods smile upon you).
Smoke is great
Many vehicles come equipped with smoke launchers and they are awesome. They add -2 to any attack against you. This is great if you need to rush a target or cross an exposed area. Just remember to reload the launchers when you have a free turn.
Use that ruler
The R key brings up a ruler. Check those ranges for movement and targeting.
Look at the terrain - Not all maps play the same
The maps are designed as a puzzle. Some will promote a very defensive approach with bottlenecks and narrow passes. Others encourage the use of long range AT with large open areas.
Before placing your units take a look at what you have at your disposal and where it might best be placed to achieve your objectives.
The organisms can be useful
They will destroy both friendly and enemy units. I've boxed the enemy in and had them waste missiles and lose units from a rampaging critter. I've also lost tanks when I haven't paid enough attention to them when they're amongst my own troops.
Some times you have to sacrifice
It sucks but some units may end up as fodder for the cannons. Rushing tanks with infantry can work wonders, especially if you cant risk losing your more expensive units.
I mentioned earlier that infantry supporting each other is highly recommended. The same can be said for tanks. Rush a target with 2-3 tanks and you may be able to take it out in one push.
Destroy damaged armour first - if it's destroyed that means one less shooting back at you next turn.
Pick your targets
This in regards to your artillery. I will typically take out enemy ATGM teams as soon as I can. Their 6 hex range and powerful missiles will lock your armour down and chew it to pieces. Luckily they take a turn to set up and only have 2 health.
Other important targets are enemy mortars, artillery and laser teams.