First of all this is my first post soo bare with me :>
Most mages know this but for those who don't it's good to read.
What to do in 5v5s as a mage:
Unfortunately in 5v5s mages are mainly Crowd controlers you're main purpose is to Counter Spell Paladins / Shammans on cast so they will be rendered to uselessness for 8 seconds.
Speck: Talent Calculator - World of Warcraft
This is the speck i prefer because you don't need improved counterspell as much in 5v5s as you do in 2v2s, or 3v3s. Therefore you can get more talents from the frost tree and the verry usefull impact from fire. Impact is mainly taken for Molten armour because it isn't affected by blessing of freedom soo it will be much easyer for you to stay alive. The armour also gives you -5% to get crited and 3% cirt wich makes it the best armour for professional 5v5s.
You need to take the Winters Chill because at start you will spam frostbolt rank 1 (or icelance after the frostbolt hit) + detect magic on the target you want sheeped. After all the debuffs are applyed you can just sheep him and wish the paladin fun in dispelling 8 debuffs! xD (frostbolt slow, 5 winters chill, detect magic and sheep). You need to do an incelance on the target once he's out of sheep soo you reset the time till the 5 stacks of winters chill fade.
This will give you enough time to cast a few offensive spells on the target getting nuked wich takes us to the next step.
How and when to deal dmg:
Idealy when you have the target (usualy warrior if you have a 2 healer setup or healer if you have a 4 dps setup) under control and your teamates are getting the main target down you wait to see the paladin casting a heal and counterspelling him on it.
For this i find this mod a must. FocusFrame | Downloads
It's a Blizz like looking portret of the player you have focus on. This will show you his hp, mana and most importantly casting.
After you have succesfully counter spelled the paladin you can make a frostbolt + icelance combination on the nuked target using the pet nova at the last second so that you are sure it won't brake untill your frostbolt hits. After that you get back to croud controling.
Another usefull mod that all your team mates should have is:
Proximo | Downloads
This one shows all your opponents after you or your team actually targets them. It's really usefull because you can select one of them verry fast in the huge croweded mess that 5v5s are. It also shows their hp % and class soo finding out when to counterspell paladin / shaman is much easyer.
Hope you enjoyed my littel guideand gl in 5v5s
Tibs