Mesmers

Being a new player and I was playing the random arenas in Ascalon today, and I saw a level 10 Mesmer/Monk who was KICKING ASS. Her name was something with the word Mirage in it and she could take on 2 or 3 enemies singlehandedly without breaking a sweat!

So now I started a mesmer and just realize how cool they are I thought they're like, weird n stuff, but their actually really awesome!

I was thinking about taking monk as a secondary but I heard that mesmers can also be very useful combined with necromancers? which of these is better for PvP?

PS, or elementalist, I just learned some elementalist spells which are quite awesome.

First of all, Ascalon arenas can hardly be described as real pvp. Either way, if you want to get an experience of the class in PvP, mesmers are generally somewhat gimmicky for arenas.

In terms of PvE, most people here would advise using ele secondary for the damage spells, but thats just because they're idiots and think fast cast actually does shit in PvE. If you wanted to play an ele, make an ele, I'd say. Anyway, as a mesmer, you'd probably want to focus on the domination line. Things like... esurge, cry of frustration, etc. Ele secondary might still be good as you could use glyph of lesser energy to power your spells.
The other possibility is illusion hexes, which pretty much blow anyway. Illusion mesmers can be decent with necro secondaries, but they suck to play and to play against. I'd say go with /mo or /rt if you find your energy is fine for the fast cast hard res. An FC flesh of my flesh owns face and a 1s full health/energy res (death pact) is just amazing!

I once fought a team that had a tank, a healer, and two elementalists in the Shiverpeak Random arenas. They killed off three of my teammates, and only our level 15 assassin / monk survived. Then the assassin soloed the whole opposing team.

Having monk as a secondary profession can do wonders, I tell you.

As samcobra mentioned, you want to move towards the domination line.

Going /E for Glyph is very common, however one of the major purposes of going /E (ward against melee) recently had its energy cost increased to prevent mesmers from doing just that. For the most part, intelligent use of Power Drain will do more for you than glyph - adding enemy disruption as well as a high net energy return (and freeing your secondary).

/Mo is only really good for the hard ressurect, however it is completely outclassed by /Rt if you have Factions; Flesh of my Flesh, or with more experience, Death Pact Signet (depending on the rest of your build) are faster and bring up your ally with more health/energy.

Illusion mesmers can really go any secondary - the buff to Signet Of Illusions make it a little more viable for playing outside the Mesmer lines.

The Mesmer primary combines well with any spellcasting secondary. Fast Casting applies to all spells and signets, not only to those within the Mesmer's native skill set.

At the very beginning of the game, illusion can be useful. Monsters with lower health fall prey to degen hexes like Conjure Phantasm very quickly. Although illusion quickly starts to lose its shine as the monsters get higher health levels, and domination becomes the way to go. Domination has a few reasonable damage options, as well as several interrupts. Practice your interruption skills - a well timed interrupt can save not just your butt, but the whole team in a tight spot. Plus, interruption is critical for PvP (and it's more fun if you know your Cry of Frustration just made someone else really scream out in frustration).

Pretty much everything you need to be an effective Mesmer is found within your native skill set. Though secondaries can help give you some other options. /Mo and /Rt give you hard rez skills. /N gives you plenty of skills that inflict conditions, and seeing as the Mesmer has a few skills that let you spread or concentrate conditions around various targets, that's something you could try playing with. /E gives you glyphs to play with to enhance your spells in various ways. But whatever you decide on, remember that a secondary is a secondary - it's supposed to help support your primary profession, not to take over completely.

In my opinion, go Domination for most of the game, and only rely on Illusion when you have no other skill options. Illusion is just not as effective as the straight, armor-ignoring damage of the Dom line. Most of the time, i just dont use my secondary prof. I dont really need it... The only time i change it is if i need GoLE, or Res Chant from Monk....

Having assassin as a primary profession can do miracles

An assassin in Shiverpeak arena? He must have max armor then.