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.