Before we begin our walkthrough, let's first go over some general strategy that will apply to all campaign missions.
1. Use all of the mission timer
All stages (save the first, the last, and the first half of Alpha 3) have a timer on them. In some stages, you'll find yourself wishing you had more time, and on others, you will think you had "too much". Trust me, there is never too much. Whether just letting your cash tick up, recycling-replacing your forces with new technologies, or setting up defenses at key locations (for base-missions anyway), you can use the extra time on one mission to make your life on further ones easier.
But mostly, the reason you want to prevent stages from ending is so that you can research all the techs you get in a given stage. Falling behind in this can lead to your finding out that you really wish you had access to something in the next stage, but you're still an hour's research away.
2. Retreat at X damage
Юниты in this game accrue experience as they shoot enemies (and commanders get it from anything their attached units shoot). Experienced units end up being quite a bit better than their inexperienced equivalents.
In a multiplayer game, this is usually not too useful, as most MP competitors know to specifically strike against high-experience units first; but against the AI in the campaign, having a force of heroic tanks is amazing.
One of the orders you can give to your units is for them to retreat if their health gets too low (50%/red, or 75%/yellow, specifically). The units with higher HP around them will protect them as they retreat. In most circumstances, Отступление при среднем уроне is the best setting.
3. Сохранить often
Like many games, saving before doing something is a good way to make sure you don't do something... you regret. This game has a number of twists in it, twists that, even if you read this guide, you will find yourself wishing you had prepared for. I recommend no less than 3 save games; one you take right at the start of a stage, one that you keep saving over as the stage progresses, and one right when you start "milking" a stage's timer after completing the objectives. With these three handy, you should never be in a position where you find yourself saying "oh crap, I needed to start handling this half an hour ago!".