How to plan a good attack on 1 hex

[Amnesia]

I will start with a few question :

Why when I try to attack a shitty position of Patton (1 russian cav and 1 tank fortified), with 4 Panzer, 5 infantery, 2 art, some air unit on support, with an attack and defense strengh at 8 against 1 on ground and 3 against 1 on air, it seems impossible for me to take the position.

But when HE attacks me, on a position where I have 5-6 good entrenched or fortified units, he attacks me with less unit, and it is magical, he pushes me like he was striking into a cream.

Same with Kru....

It is really really really frustrating and cut me the motivation to play..
Sometime I tempted to think that they are cheating.

Actually I think there is some critical and subtil detail to know when we plan an attack, and I would like to get some advices.

thanks.