Slitherine

This must be a very basic question, but I really don't know how this goes:

What happens in 2nd phase of battle when defender shoots attacer?

When I'm attacking, do I benefit from the terrain bonuses of my own hex?
Is it safer to attack from defensive bonus hex, because then defender can't hurt you so much.

Terrain bonus is a modifier to the defender. So if you attack then your attack strength will be reduced by the terrain in the defending hex. Your current strength is shown when you hover over the unit and that will be the base when attacking the defender. You can see the defensive bonuses by hovering over the defending hex after the attacker has been selected.

So attacker is not a "defender" in the second phase of attack when defender is firing at him, he is only a target. So this applies to rivers, as well. River doesn't reduce defenders ability to inflict casualties to attacker in the second phase.