I have a couple, but one really sticks out in my mind...
My friends and I were running some Fractals of the Mists dungeons in Guild Wars 2. This was before the Heart of Thorns expansion, so things were harder. I was on my elementalist (spec'd at the time for healing).
We had gotten this boss down to his last phase. It still had quite a bit of health left, however. I was the last one alive.
Since I was spec'd healer, I kept myself alive as I whittled this boss down slowly. But eventually, I'd finished the sucker off by myself.
The moral of the story is, thank your healer :P They save the group.