My most important advice (which got me from second to third difficulty level) - don't buy mercs for two weeks. Buy them for 24 hours, or for a week, and get your hands on non-standard income (I mean, go to San Mona and do ring fighting - you will need Blood to win all without savescumming - do the quest with leather shop (I won't tell anything more about at, as it would be spoilerish, but you can easily find walkthroughs for that), and kill Kingpin and steal his monies (that would probably need to wait for a moment)). This way you will be able to pay for above-standard mercs at the beginning, so you will be able to get Chitzena, fortify there, and get Drassen. As for the counterattack (it could be switched off? Never even noticed that ^^") - don't defend against it. It is futile. As soon as you get the mine, fall back to the airport, wait for the counterattack to come, and then counter-counterattack it. It's much easier that way.
Always keep your mercs behind cover. Never rush. Be patient. Be patient (I know, I'm repeating myself, it's that important). All the scopes on all the weapons (x4 scope is best all-rounder, x10 on the sniper rifles). Anything firing rifle bullet is better than everything firing pistol one (there are exceptions, but you won't notice them anytime soon, probably).
As for the mercs, I'm not saying I have a good setup, but I always get Barry Unger (he is cheap and repairs well at the start, and he learns extremely fast), and at least two high-marksmanship people (Buns is good for that, as she can patch some wound and is cheap, for example). I only used Steroid once, never went back to him, actually. Also, try to avoid getting conflicted mercs. It leads to problems.