As you may know, a Codex Team will consist of 4 people, these 4 people have a set skill list to choose from which is referred to as the "Deck" or "Codex". For a list of the current skills available in the Codex Arena, go here.
In most cases, teams will be setup up with 1 Heal based character, 2 Caster/Midline based characters, and 1 Melee/Frontline character. However in most cases this is extremely dependant on what the Codex includes, many teams will adjust to what the Codex includes, whether it is a lack of Good Healing skills (meaning a secondary healer such as a Ritualist being taken), or the lack of good Frontline material (in which case 3 Caster/Midline characters could be taken).
Setting up Healer Builds in Codex
The main reason teams survive for decent lengths of time is because of Monks, these are essentially the primary choice of healers. However with a Sealed Deck it may be hard to come up with a decent build to keep your team alive. These are the basics that a Monk should have in Sealed Deck:
- Power Healing (through Healing Prayers most of the time)
- Hex Removal
- Condition Removal
- Prots (although they may not be needed in defensive setups)
- Self Defensive Skills (skills from the Tactics/Shadow Arts lines work best).
Anything that your Monk cannot take due to the confined Codex should be made up with through other Team Members (Midliners mostly).
Ritualists also have an oppurtinity in Codex to use their Restoration Magic to support/heal allies. Depending on what kind of Monk skills are in the Codex, Ritualists often fill supportive roles to assist monks, although they can be used as a main healer.
Midliner Builds in Codex
The "Midline" in Codex Arenas is usually a mix between support, shutdown, and offense. Elementalists, Mesmers, Necromancers and Rangers are often the best choices for Midliners, due to their range of Offense, Support, and Shutdown. However this is all dependent on the Codex, if some profession do not get key skills for their profession on the Day, they will probably be best left out for that day.
As mentioned above, Midliners will often have to pick up the left overs of what the Backline (Monk or Rit) can't offer, be it Hex Removal, Condition Removal, etc, and will have to deal with it as best as possible with the skills they have available.
Frontliner Builds in Codex
Like in all areas, Frontliners are often left to deal a majority of the Team Damage. Frontliners can consist of Warriors, Dervishes, Assassins and occasionally Rangers (however that is very Codex dependant). A Majority of the time, Warriors will be the best choice as they can still do a lot of damage even with "lackluster" skill choices, however Assassins are very dependant on what attack chains they can use, and Dervishes are dependant on a few Overpowered Skills.
If there are very few skill choices available for frontliners it won't be rare to see them replaced by a third Midliner.
It is always important to discuss what each team members build should be made up of, because of the Codex, it is important to have as much as possible in your team, not just as much as possible from 1 person. As long as your team can cover all the bases defensively and offensively, then there is a good chance of a long lasting Win Streak.
It is important to remember that the team you will be facing are forced into using the same skills as you are, so if the Codex has a lot of Hexes, make sure to use all available Hex Removal skills from the Codex. This will work the same way with Conditions. If there happens to be good Warrior/Assassin/Dervish etc build available in the Codex, take an adequate amount of anti-melee to deal with it, obviously without hindering your own teams damage output. In short, be wary of the Codex, read it and try to predict what most people will be running, in turn make sure you can counter/beat what most people will be running.