Group Arena is a team of 3 people fighting another team of 3 people from other servers.
The match up system is based on highest level. This may not always be the case but majority of the time you will be matched up against someone of equal or greater level.
From my personal experience it appears to be something along the lines of 40-45, 45-50, 50-55, 55-60 and so on.
Your average fights should be against players within a 5 level range. But there will be times you fight someone who may be 20 or more levels ahead or below.
Now, below is just some common team formations with a rough idea of the tactics behind it. they tend to have greater success because there is more team work involved than simply just casting your AoEs and hoping you win...
2 Knights, 1 Mage
Reason this works? On first turn the Knights cast Whirldwind if possible with the mage casting Rain of Fire, any mages or archers hiding behind the tanks will take more damage.
Knights can then proceed with Ultimate Slasher to pick them off. Quickly turning into a 3v2 fight. or 3v1 fight.
If Whirlwind is not a possibility due to no clothing, then it's going to come down to hoping the backrow is weaker to Matk and the Knights can begin picking them off with Ultimate Slasher.
2 Mages, 1 Knight
One mage will be preparing for a Blessed Light, while the other deals a bit of damage with the Knight, when it is needed the mage will cast Blessed Light and heal the team. Then he will resume with Rain of Fire and so on.
The other mage will then prepare to use Blessed Light as well for the next time the partys HP drops too low. This will exhaust your opponents rage and keep your team alive.
Knight will do his thing and try pick the other team off.
This formation is can be quite painful,if 3 Archers can start off with Lunatic Fire then it's going to quickly leave the mages dead, or close to it. Following up Lunatic Fire with Multi Shot should finish the job.
Fighting a knight with a lot of Pdef is going to be a pain, but if you have enough HP and Pdef of your own, along with attack this should be a 3v1 fight and you can bring the knight down.
This one is probably my favorite, so many possible strategies and fun ways to kill your opponent if you have the rage.
But for the quickest way, you could all use Whirlwind followed by Ultimate Slasher. It'll make quick work of any mages and archers.
After that it shouldn't be too much trouble bringing down whoever is left.
Early on in the game, Mages are king of the battlegrounds, so if you're a mage and on a newish server (Below 50, possibly 60) This would probably be the ideal team for you.
Most classes defenses are lacking in the early stages, and for good reason, so if you've built up enough Attack, then Rain of Fire and Meteor Destroyer from all 3 will make quick work of other teams.
Later on, this is no longer viable, At least one mage will have to use Lightning Bolt and build up rage so when the parties HP drops, they can use Blessed Light. I'd prefer if all 3 mages had Blessed Light ready and take in turns using it as the time comes.
All 3 should be in the same row, splitting the damage as equally as possible. Unless of course there's one with enough HP and Defenses to be a clear winner for passing off as a tank.
1 Archer, 1 Knight, 1 Mage;
Probably my least favorite, it doesn't really work in my opinion. Archers are generally weak in defenses and make up for it in HP. But later on this doesn't help much when they get blasted by mages or knights.
I've found Archers go down too quickly, mage is unable to heal in time and then the mage quickly goes down soon afterwards. Leaving the Knight all by his or herself.
There are a few other combinations I won't mention at this time. They should be slight variations of the above tactics.
Archers pretty much get the short straw since they are more designed for 1v1 fights, 3 Archers can be a terrifying team to face. Especially if you're a mage.
Now since Insignias are up for grabs in Arena, I strongly advise, if the situations arises, don't hesitate to use a rune.
A mage using Rage rune to quickly get out a Blessed Light could save the team, or perhaps a Thunder rune to quickly finish off someone close to death can take em out before they get off another attack.
At some point you will want to make PVE equipment if you haven't done so already, and the one thing most always lack is the insignias. So get as many as you can while you can!