Let me share my strategy ^^
1st, You need 2 sorcs, 2 priests, 2 Epicly geared tanks (2 knights would be good) and 4 DPS.
(It?s all about the gears bro don?t go blaming your priests or pt members when u die when you?re obviously under geared)
2nd, You want to know your positions ask a sorc to open Land Recovery on himself. Tanks and melees should be standing in front and in range of LOR others like ranged and Healers should stand behind of Sorc at the back part of LOR practice a few times before you go.
3rd, When you guys got the hang of the positions. Move together as a group and ask knight to initiate the battle. All party move as a group and stay in formation as Knights moves forward slowly. This is because the boss has a skill that makes you in movable so if you move as a group this won?t be a problem.
4th, Decide who is the better tank. The more buffed up tank should be taking 3 bosses at once. While the other tank takes 2. Knights should only use their AOEs throughout the entire battle. Make sure no dps surpasses knight?s threat. Sorcs LOR should be always be active deluge if any big problem arises.
5th, Take our their Wizard 1st as it?s AOEs will be the most annoying. After wiz is down it should be smooth sailing.
This is how I beat the last boss of arena. Feel free to correct me or add some of your own personal opinions.