Persuade Arthur to go up to her and say "I've been told that you want to hold my sword Guinevere." *sexy eyebrow raise* "Is that true?"
Tell her you know that she smells Merlin's neckerchief when he's not around because she likes his scent.
Mimic her words to Arthur. "Because I have faith in you." Then descend into a lovesick declaration. "Your eyes! Your blonde hair! The way you hold your sword!"
Jump out and call her a witch at every possible opportunity.
Sneak up behind her and play a taped recording of Arthur saying her full name, like he did the time in Kanen's village.
Tell her Merlin/Arthur doesn't like girls with curly hair.
Tell her Lancelot has returned and say – "He really wants to see you Gwen. In fact, he's been hoping that...you could introduce him to Morgana."
Plant some of Arthur's underwear in her room, then accuse her of being a pervert.
Tell her it's a good job she's not around Gaius' patients a lot, if she kisses them every time they recover from being poisoned...
Ask her if Merlin has ever walked in on her kissing Morgana.
Ask her if she thought Lancelot was any good in Heroes.
When Gwen is within earshot, tell whoever you are talking to loudly that you knew Gwen couldn't possibly have magic...because she simply isn't pretty enough.
Plant her diary in Arthur's room, open at the page where she has written all about how much she likes him.
Say to her "Look, Gwen, when Arthur said after your father died if there was anything he could do for you, to let him know, he didn't literally mean anything. Stop taking advantage of him!"


