When you have taken drassen, and talked to father walker to set up a (hidden but functional) resources route to omerta, start training militia from the income from the mine. This is a critical moment, once again, especially on hardest, as there are typically enemy soldiers around the city. And they can attack at any time. Use your high leadership IMP merc, combined with Ira (the free merc) to train militia, not allowing sleep until they have to (click on the bed symbol in the strategy window to wake them up).
Keep moving militia to the city window where the enemy soldiers are next to, to better the chance of defending when or if they attack. You CAN engage in battle yourself, helping the militia, but it is usually a bad idea, the militia perform better when automatically engaged (auto-battle), and your time is better spent on other things.
When you get a proper garrison set up at drassen (50+ regular, 60 max), which takes typically up to 10 days, you will most likely have built up a cash sum to hire a team of mercenaries. Again, try to get lynx on the team, he has the mini-14 hunting rifle which is almost essential to take the next town, which should be chitzena, way across the map. In this game, a few good rifles are worth infinitely more than a group of hand gunners.
That is the main tutorial. When you get going in this game, starting to encounter elite soldiers and defeating them, it gets real exciting - you get m-14, fn-fal assault rifles, m24 and dragunov sniper rifles, spectra armor, sniper scopes.. you can hit enemies from wayyy afar ^^ it takes high marksmanship and great equpment to do it, but you feel damn near invincible when doing that ^^ But, towards the end of the game, it becomes extremely difficult ^^ the enemies wastes no time launching rockets on you and if they hit, it's instant KO, or at least it is just as well to reset, since it will severely wound your mercs.
The key is to keep mercs in a line, so that when you see an enemy, you can instantly kill them, giving them no chance to see you and hit from a hidden position. The more you allow them to get a view on where you are and position yourself, the higher the chance of receiving a mortar shell or rocket. They also shoot with high precision and high caliber bullets, even if you wear good armor they will cause damage.
But with expert tactics and real patience, it can be done with minimal losses. That is why the game is so great, it teaches you to be patient and tactical. And teaches the importance of preparation. Going into battle with damaged and unrepaired gear and guns is no good. You put the best mercs in front and the others right behind, ready to go ahead if those in front get damages. Function as a super-slick functioning team. And pull back and retreat if that is necessary too, heal up, come back another day.