Mesmer/???

I just started reading about Guild Wars so i'm not so familiar with how things go down here but i was wondering what is a good secondary class to go with a primary mesmer. I want to do a lot of damagae targeted primarily for PvP. I saw this one video of a mesmer (not sure of sec. clas) just ripping through tons of people in PvP, im not sure of the secondary class but I'm pretty sure i saw a fire spell in there omswhere so maybe a fire elementalist. Well if you could give me any input on a great class to go with a mesmer for tons of damage please do. Take care :>