Something that causes a lot of harm can be called "devastating". You can use it to talk about a battle or attack in war:
The Battle of Galipoli was devastating for the Allies.
You can also call a punch or other blow "devastating" in a fight between two people:
Oh, that kick was absolutely devastating!
One other way that we use "devastating" is to talk about something that caused emotional harm:
My dad passing away so suddenly was just completely devastating. It took me years to process it.