Different monsters fight different, each have strengths and weaknesses. You can beat that single ogre or troll who's looking strong or deadly, and get mopped by an army of darklings flagged as weak.

It is not an accurate measure of how well you will do in battle. Their strength indicator is more tied to level / numbers. If you get surrounded by groups of 4-5 mites they will instagib your hero or whatever unit was there. Swarm bonus etc.

You need to learn how to fight against different monster types, what are their key units, what to look out for etc. You can be a level 20 and get mauled to death by a level 5 bear because of bad rng on your dodge (if any) etc. Of course the bear is flagged as weak since is has 12 hp and low attk, no defence etc. That doesn't mean he should just flop over you or hit for nothing

Yes that's VITAL to inspect the enemy you're about to attack (or if you get ambushed, spend first round checking what mix of units and abilities they have before deciding a course of action or retreat if necessary).

I remember in a game couple days ago I was in early game, with a level 5 hero and level 4 spearmen / militia and a golem lvl 3. And run into a simple cave bear pup. Level 2. 8 or 10 hp can't recall. And he went straight at my militia and mauled their @$$ into oblivion killing them with nothing on my side I could do to prevent it. 

But that's one of the best things about this game. If you could predict and control EVERYTHING, the game would quickly lose interest and become plain boring, since you had 100% control of everything. The most fun / challenging / surprising / exciting part is how to adapt and react to unexpected situations. Like enemies getting a crit. Your mage missing a counterspell. That little bear pup getting good RNG on his maul chewing your unit down.

RNG is good for the health and replay factor of the game. If everything would be the same (aka unit A beats unit B no matter what) it wouldn't be fun

Now that we agreed, let's go smack that Troll Shaman group, he's blocking the road and I can't park my horse!

Troll shaman group?  Just bring Izzy-whats-his-face along and let him 'coal-fire-em all.  lol

Yes, them trolls love to smack upstart fire wizzys down... lol.  Best thing with trolls is lots of cheap summoned beings to keep them busy - while you range attack them, and/or hit them (one at a time) with all your decent hitting troops.  hey, sometimes you even survive.  Oh, if you get the giant cat quest, don't bother giving him 'eviscerate.'  It ends the 'maul' attack sequence (sad).  But he is great against trolls, especially with a bit of 'dodge' added on.

Wait, I knew break didn't work (though I was under the impression that they'd fixed that one), but shadow strike doesn't do anything either?! What a gyp! I wondered why my Assassin wasn't doing much of anything in battle.