Italy, yes Italy, taking on a giant Gray by himself and winning, even though he gets injured.
England killing one of the Greys all by himself. It comes at the price of his eyesight, but still.
When England and Japan link their powers together to unleash a powerful joint attack. It was so epic that this troper literally fist-pumped the air and cheered. Hell, even the picture◊ they used was awsome.
Spain in episode 17, single handedly defeated at least 3 of the greys, while defending Romano!
Not wanting to leave a damper, but he did them separately and they were the earlier time-loop's, weaker monsters, so it's not really exactly on-par with England's moment... It's still pretty awesome though!