It was mortifying, so I wanna get even.
Catwoman! You are so evil! I’ll get you back soon!
I ate Koichi's favorite yogurt to get back at him for eating my pudding.
When you return the doings of another person, you're doing it to get even (whether it's a nice thing or a mean thing).
The readings are a bit weird, here. 仕 gets its on'yomi reading... you should be okay with that. 返し gets the kun'yomi reading, though, which is the reading you saw with the word 返す in level 9. You can tell this part uses the kun'yomi vocab reading because it has that hiragana attached to it. Just remember to distinguish between the two parts of the word to remember the readings.