If love be rough with you, be rough with love. Prick love for pricking, and you love beat down. Mercutio is making a sexual reference.
The game was ne'er so fair, and I am done. Tut! Dun's the mouse, the constables own word. If thou art Dun, we'll draw thee from the mire. Mercutio is saying that if Romeo is actually "done" because of his lovestruck behavior he will make himself to sink into mire, or his poor state of mind.
Give me a torch. I am not for this ambling. Being but heavy, I will bear the light. In this light is the pun. Romeo right now feels "heavy" as in his mood. so light refers to the light of a torch, but also Romeo is lacking lightness in his mood.