Taking on the odds
 

In KBTL you were able to defeat enemies who were strong, very strong and lethal without zero losses on hard.

Yet in KBAP it seems that one has trouble doing that with "equal strength", on hard, with no losses, is there a specific class or special tactic that makes this possible or easier?

I'm as paladin on lvl 8 now and I can't get anything done despite having 29% critical on almost all my units and having 4 defence 4 attack 5 intellect, I basically have nowhere to go now...

I'm not having problem with equal strength on hard yet. If you are than you are lacking two or three the following:

Summoners
Tanks
Ressurectors

I manage to take everyone who is not invincible. If they are invincible they have to be of certain types of troops for me to defeat them.
Heroes who has invincible is a pain, but it can work.

I Remove a great deal with a 1800 point fireball and 700 point fire arrow at the start.

I make sure to both poison and give burn to the strongest as fast as possible, therefor making sure they loose a lot of HP each turn.

Snakes are some of the best troops you can have in my opinion, since they are great at avoiding counter attack!

Oh, you meant zero losses, no, i can`t do that on such hard levels.
I am not even gonna try, enough pain getting through on hard with mage as it is :-P

The problem are those damn heroes who throw fire arrow or poison skull and ruin everything..

I think I'll go with 3 units from now on, Guard Droids and Repair Droids divided into 2 groups so they can repair each other...I think this will work.

In my opinion 1 stack of guard droids and 1 repair droids, split in half will suffice. Back them up with Archmagi (Magic shield on the guard droids, since they will be your tanks) and Inquisitors (in case any archmagi or inquisitors die) and you have a party that can heal eachother at will.

Of course, I completely forgot about Archmages and their magic shield...and I already have lvl 2 stone skin.

And of course, magic shield is one of those medal award requirements...silly me, I was expecting to find that spell.

How come with over 50% of critical chance on almost all your units, critical strikes rarely happen, Sea Dogs will far more frequently dodge attack with their 20% evasion than I will score critical strike with over 50% chance?

Because 50% critical doesn't mean 1 in 2. It's 50% for each attack, so it may either happen or not. It's all about luck. In some cases 50% may mean 1 in 5, or even more.