Blood Coliseum is a Level 45+ dungeon with two different modes, hard and normal. Entrance can be gained by speaking to the Mysterious Wanderer (Dungeon) in the Shrine of Kithara. The dungeon consists of 4 waves of mobs, each finished by a boss. The enemies will start by spawning at the top, then the right, then the bottom, then the left, and then they will spawn in the middle with a boss. Not all of the enemies need to be killed in each wave, however, Succubus spawned in waves attacking from the sides must be killed. The bosses must also be killed. The 4 bosses, along with strategies, are as follows.
Boss 1: Midknight
Midknight is a large, sword-wielding skeletal knight. He will switch between 2 "modes": In one mode he is vulnerable to physical attacks and highly resistant to magic, and in the other he is vulnerable to magic attacks and highly resistant to physical attacks. Regardless of mode, he won't cause too much trouble, but if you have a party, having 2 DPS, one magical and one physical, can insure he dies a quick death.
Boss 2: Bonefletcher
Bonefletcher is the second boss, a large skeletal archer. He hits very hard, and occasionally uses a buff that greatly increases his attack speed. This will probably be the toughest boss to fight, but there is a tactic that can be used to lessen his blows. The mobs that spawn with him occasionally use a "curse" that decreases magic defence, but increases physical defence. Because Bonefletcher doesn't have a magical attack, leaving one such mob alive (and hostile to the party tank) can help him/her survive much longer.
Boss 3: Apothecar
Apothecar is similar in appearance to Neratul from the Revenant's Vault dungeon. In addition to regular attacks (not sure if they count as magical or physical, I'll have to look into that), Apothecar also wields a curse, Viper's Touch II, that drains HP every second and reduces the efectiveness of healing on the cursed target. The best way to deal with this is to have a Priest with the Pure at Heart skill to remove the curse as soon as it is cast, because it is the source of most of Apothecar's damage, and the reason that it will be harder to heal it.
Final Boss: Vicelot
Vicelot is the easiest boss in the dungeon. His attacks are not all that spectacular, and he doesn't have a terribly large amount of health. If your party got through Bonefletcher and Apothecar, this boss shouldn't give you any trouble. He has a chance to drop a Brilliance Shard upon death.