There is still tons to do on Roak, including fighting in the Coliseum and exploring the many side-quests, but we'll leave that to your discretion.
Edge should keep his distance and focus on fighting his henchmen, and then attack from behind when he has the free time.
When this happens, you should try to spread out and used ranged attacks (hope you leveled up a well-rounded group). When his life is about half gone, he'll begin attacking with a powerful and hard to avoid beam attack that can mess you up in a hurry. He can summon henchmen, which you should focus on fighting with Edge so that everyone can concentrate on Tamiel. The first half of the fight is deceptively easy.
To start with, you'll fight two waves of his goons, which should be nothing. You should be at least level 38 or so, and even then it's a challenge. Tamiel is the most difficult boss yet, so don't try to force this under-leveled. Take the stairs down and there's a save point. There's a heal point, and a couple chests. The water lowers, and you can now reach the lower level. The correct order to light the torches is: southwest, north, south (in hallway), east, northwest (in hallway), southeast, northeast (in hallway), west. When the statue appears to point to them, it is actually pointing at a more distant torch in the hallway outside of the room. The already lit pots are used as red herrings. There are eight torches, four closer to the center at the cardinal directions, and four in each corner, but some are already lit.
On the B3 level, there's a series of walkways around a large water-filled chamber. If you go through and hang a left, you'll trigger an event and open the way to the next floor down, but you might want to stop off at the crystal further north to charge your ring first. The correct sequence is northwest, southeast, southwest, northeast. The twist this time is that after you light the first torch the statue will indicate the rest of the sequence without stopping, so watch it carefully. After that, you'll come to another torch puzzle. You'll need to use the Light Ring to take down a shadow barrier. Head through the southernmost exit on the east wall.
When all the torches are lit correctly, shallow staircase will lower to the sides of the room. They're not arranged on a neat grid so it's difficult to describe the exact sequence, but just make visual contact with at least three of the statues before lighting each one, and you'll know you have it right. There are four statues and nine torches to light. To start with, you want to make your way to the southeastern part of the stage and take the stairs down. The good news is that the crystals themselves respawn if you leave the dungeon and then come back, so don't be too afraid to explore. Even worse, the large growing crystals you've seen only provide a single charge each. The light ring only holds one charge at a time, which gets pretty annoying. Other than that, it's a pretty ordinary fight, and not nearly the end of this stage. Myuria is great here, since he's weak against Thunder, Fire, Water, and Darkness attacks. Head north on the west side of the map and you'll encounter the Guardian Beast. Heal your guys and make sure they're ready for a boss fight. There's a torch in the southeast corner of the room, one in the southwest, and one in the center. You have to look at the conjunction of where the two are pointing to clear up any ambiguity. There's also a door leading south as well, so this puzzle isn't even necessary. The next room has a similar puzzle, but with only three torches. If you do it wrong, a band of Fire Corpses will attack and you'll have to try it again. If you did it right, a door on the south side of the room will open. Light this one as well, and so on until all four are lit. There's a statue holding an axe in the center, and four torches in the corners. On the first basement floor, go east two rooms.