NOTE - This except was taken from The Key of Goetia (full credit goes to ElysianField)Summary of Arena-type Events
For those who haven't participated in an arena-style event, here's a summary of how they work.
First, you assemble a team of five units, limited by 100 deploy points. Whenever you battle, two active, random allies will also join you.
There are two types of battles -- event/quest battles, where you face NPC setups, and PVP battles, where you face other players' group setups. PVP battles give BT and victory points if you win; event battles give BT points. Occasionally during event battles, you will come across a boss and have the option of fighting it -- if you fail, you can use a potion to continue; otherwise you lose.
Battles consist of 30 turns split between you and the opponent. Units take turns attacking based on speed and other skills that come into play. The battle ends when one player's stamina completely depletes or at the end of 30 turns, awarding the win to whoever has the most stamina left.
Each of the unit skills has a chance to activate during battle, and each has a certain effect:
Attack Boost: For a certain number of turns, the boost actually activates and boosts the appropriate units' stats.
Poison: Poison applies a "damage over time" debuff to your opponent, and a very slight heal-per-round to yourself. Poison lasts a certain number of rounds and damages your opponent whenever they take a turn to attack.
Slow: Slow decreases the opponent's speed, occasionally pushing some of their units further in the line-up and yours up.
Knockback: Knockback is, in my opinion, a better form of Slow -- it completely removes the opponent's next unit and places them at the end of the current attack cycle.
Area Attack: When activated, damages increases by 1.75 the unit's damage.
For a complete guide on creating a decent team go here
For advice on your team formation on general advice regarding arena team team go here