Spawn of Itzl is a neutral Lizardman unit with this action: When this unit attacks, destroy target damaged unit in the defending zone.

If I will attack with this unit must the opponent opposed in combat (defend), before I can use his abbility? Or I can just triger this abbility right after I willl declere witch zone I'm attacking?

I think that I can just use it right after declaring witch zone I'm attacking. But I'm not sure. Who do You understand this?

"When this units attacks" is the moment this unit is declared as an attacker, so as soon as you declare this unit as an attacking unit you can use the action, before defenders are declared. You can be sure .

Yep! same way Wilhelm works. Move them away before they can even do anything in battle.