So do i have to resolve that shadow card dealt to patrol leader still, if its damage has been canceled?

What ?

What is your problem exactly ? could you describe what cards are involved and there states ?

I cant get that picture of that card, but Patrol leader that attacks to my hero has ability that " Forced : before patrol leader is dealt damage, discard top card of the encounter deck, if the discarded card is enemy, cancel that damage. So my guestion is that will shadow card activate that was dealt at start of the combat phase?

The shadow card is irrelevant here. Here's the sequence (assuming Patrol Leader is the only enemy you are engaged with):

1) Combat phase begins. Deal Patrol Leader a shadow card.

2) Patrol leader attacks. Declare your defender and exhaust him/her.

3) Reveal shadow card and resolve effects.

4) Calculate combat damage and assign the damage, if any, to your defender.

5) Your turn to attack. Declare an attacker against Patrol Leader, and exhaust him/her.

6) Calculate combat damage.

7) Before placing damage tokens on Patrol Leader, discard the top card from the encounter deck. If that card is an enemy, cancel all the damage. If it is not, go ahead and put the damage on Patrol Leader as per usual.

Oh no i have been played totally wrong with that card! This is so embarrasing XD

The shadow card is irrelevant here. Here's the sequence (assuming Patrol Leader is the only enemy you are engaged with):

1) Combat phase begins. Deal Patrol Leader a shadow card.

2) Patrol leader attacks. Declare your defender and exhaust him/her.

3) Reveal shadow card and resolve effects.

4) Calculate combat damage and assign the damage, if any, to your defender.

5) Your turn to attack. Declare an attacker against Patrol Leader, and exhaust him/her.

6) Calculate combat damage.

7) Before placing damage tokens on Patrol Leader, discard the top card from the encounter deck. If that card is an enemy, cancel all the damage. If it is not, go ahead and put the damage on Patrol Leader as per usual.

Just to add to GrandSpleen great explanation:

That sequence may be different, if the defending character is Gondorian Spearman or have a Spear of the Citadel attached.

In this case, between steps 2 and 3, you have to do the step 7.