This strip, full stop. Nanase trying desperately to save her best friend and lover's life with the only spell that can possibly do anything to help, essentially eating away at her own life force every time she casts it, and having it fail time and again is nothing short of heart breaking for This Troper, regardless of how often he rereads it.
And let's not forget that she feels EVERYTHING the doll does. So, everytime the doll is killed, she feels it. And her doll is being cut in half with a battle axe. Most people wouldn't be able to withstand the pain of that once. She did it twelve times in a row, in quick succession. To the point where she collapses, but keeps going in the hopes of reinforcements that may or may not come.
"God...help me..." She fully expected to die in the attempt to save Ellen. She kept going anyway.
In fact, the only reason she stops? She gains a spell that allows her to take the form of a Guardian Angel. And remember, spell casters in the series get spells that relate to who they are. Taken into account her actions, it's no surprise.
Tedd: You should have told me where you were going. Grace: I didn't want you to worry. Tedd: But you lied to me. Grace: It's okay to lie to people if you think it's for their own good! Tedd: Who gave you that idea? Grace [unhappily]: Everyone I have ever known ever.
This right here. Diane realizes for the first time that she has biological family. It puts her previous denials into context and makes you realize that they're not her being dense, but actually her being in denial. Thinking about it, she hasn't had that all her life, and now at long last she does.