痛い！ is the exclamation of pain. グーで殴る ('to punch with a closed fist') uses グー (from ジャンケン — rock-paper-scissors, where グー = rock = closed fist). よせよ is the masculine casual imperative of よす ('to stop, to quit'). The repetition of 痛い and the exclamation marks convey frantic urgency. Clearly a comedy or slice-of-life scene.