Your frontline is what you use to take hits. Your backline is what you use to deal damage. Sometimes these can be the same unit, but generally, you want them to be different units.

Specializing your frontline to deal more damage or your backline to be more durable can sometimes be useful, but it's often better to specialize your frontline to be more durable and your backline to deal more damage. If you are frontlining Sledgehammers and your opponent is frontlining Crawlers, it might be worthwhile to buy Mechanical Rage for your Sledgehammers to make them more effective at blowing up the Crawlers... but generally, you should avoid buying upgrades for dealing damage to your opponent's backline units on your frontline units, as they likely won't live long enough to target your opponent's backline.

Be aware of what units your opponent is using to deal damage, and try to pick something for your frontline which is effective against them. If needed you can create a new frontline by putting units in front of your existing units.

As your and your opponents units die, you both create 'new' frontlines- try to make sure your 'new' frontlines are good against whatever units your opponent has remaining. For example, if I see my opponent is backlining Mustangs, I may put Arclights with Armor Enhancement between my frontline and backline to tank them even once my frontline folds.

When you buy a range or movement speed upgrade, be sure to consider how this affects your frontline and backline. You can often switch up your frontlines and backlines with these upgrades, and doing this intentionally can be quite powerful.

Units get experience when they kill other units. Simple enough, right? Each level of rank increases the units stats by it's base value.

Killing a tower gives the unit which killed the tower a large amount of experience.

Units which are higher rank are worth more experience.

Units you kill with damage which isn't direct damage from another unit split their experience between all living units on your side. This means fire, missiles, sentry rockets etc will split EXP of the units they kill between your other units. Oddly, units killed by friendly fire from a Rhino Final Blitz do not award experience to the opponent.

Units at full EXP cannot gain EXP toward their next level, and do not share EXP they gain with other units. Rank up your units whenever possible if you're looking for Rank advantage.

Units gain EXP when other units are killed very close to them(roughly 30 meters?). This means if there are Crawlers sitting under Wasps, and the Wasps die, the Crawlers will gain EXP. If there's Crawlers on top of a big tanky unit, and the Crawlers get shot by Arclights, the big tanky unit will gain EXP. This works even on tower kills- so, if you've got a bunch of Crawlers all hitting a tower, they'll split some of the EXP.

Percentage Attack and Health increases stack additively with each other. This means the Mustang +50% attack upgrade and the Research Center +10% attack upgrade gives the Mustang 160% total attack, not 165% total attack. This includes upgrades such as Heavy Mustangs which give % increases to units.

Rank affects a units base stats, and is not applied as a modifier. So a Rank 2 Mustang with both of those upgrades would deal 320% of the damage a normal Mustang would deal.

Percentage Attack and Health decreases, however, stack multiplicatively with other attack and health modifiers. This means you cannot 'compensate' for % decreases with % increases all that effectively- a Mass-Produced Sledgehammer will have 425 attack, or 70% of a regular Sledgehammer's 608 attack. A Mass-Produced Sledgehammer with a Firepower Control System doubles it's 425 attack to 851, or 140% of a regular Sledgehammer's attack. That's still a very substantial difference(and why cost control specialist is not great.) Multiple % decreases stack multiplicatively with eachother as well, fortunately, so you don't need to worry about ending up with Sledgehammers with negative HP values.

Some units are faster than other units. Some units have more range. Both of these are significant factors to when a unit 'engages'- when it's shooting at something, or when it's taking fire from the enemy. You usually want your units to all engage at similar times, especially early-game. Simply ensuring your entire front-line engages at the same time will go a long way to helping you win more games.

Units with more speed will run past units with less speed. This... seems fairly simple, but a lot of people seem to not quite get it. A fast unit like Crawlers, Steel Balls or Rhinos will run ahead of a slow unit like Sledgehammers or Fangs, even if you place them parallel. In order to get a better engagement, your fast units should usually be placed behind your slower units.

Units with more range can be more safely placed forwards. Marksmen in particular are a good example of this. You can put them right behind your frontline- or even in front of your frontline if your frontline is fast enough- and they'll still be fairly safe. You absolutely want your long-range units to be firing as the enemy is firing on your own frontline, or even before.

Later on, you may want a counter you have set up to engage at a more specific time- think a unit of Fangs a ways behind your line, which walk up once your opponent's Arclights are dead and distract their Marksmen from your more valuable Phoenixes. These sorts of timings are hard to see, and even harder to actually set up, but very worthwhile.

Some upgrades change a unit's Speed or Range. These will change how your units engage. Be aware of them.

I will be adding more to it as I play and learn more.