Ok I'm in teth'ella and looking for the rheairds and I keep talking to..

Alright, I was stuck here too. I lost my memory cards, so I can't give you specific directions to find the door, but I remember that the answer was something like the giant tree's life is the source of mana (or something like that, it's been months). Just go to the room with all the lifts and there's an intricate puzzle that you must finish, but that's where the door is.

Now the final room. Talk to Orochi now that you have the pass codes.
Then, from the main room, enter the door at the right and go north into the
Next corridor. Then enter the door at the left in the next room, and once
In the other corridor, head north and go up the stairs leading to a room
With a save point. Use the Memory Gem on it and save your game. Go up the
Stairs northeast for a chest with Silver Mail. Then, grab the 2 blocks in
This room and push them northwest into the 2 holes against the wall. Then
Go upstairs at the left and down the stairs leading to where you pushed
The 2 blocks. You can now cross to a chest with Lavender. Also hit the blue machine with the green rings around to activate it
With the Sorcerer's Ring (shoot hit diagonally). Then, go back down and pull
The blocks out of the holes, then push them next to the stairs in the
Southwest corner of the room. Place the 2 blocks just next to it, then go on
The stairs and from the landing of the stairs (in the middle), walk on the 2
Blocks you placed just next to it and open the chest hidden behind the stairs
For Aqua Cape. Then, pull back one of the blocks out of the alcove and push it right
On the lift near the machine you activated before. Place the block on the top
Right corner of the lift. Then, while on the lift, press the switch at the
Left of the lift to raise the lift. Then push the block at the right of the
Switch above and press the switch again to go back down. Then use the stairs
At the left of the room to go up to where you left the block next to the
Switch. Pull the block to the right, then push it down at the end of the
Platform where there is a hole in the barrier. It will fill up the hole so
You can cross over. Stand on it and shoot the blue machine with the green
Rings to activate it. Go back down there and stand on the lift at the right of the room.
Press the switch next to the lift to go up. Once in the area above, go left
And shoot the blue machine here. Then go north and open the chest there for
Beam Shield. Then go down using the lift with the block on. Once down, push
That block into the hole in the barrier at the right. It will fill up a gap
So you can reach the chest with Ex Gem Lv 3. Now, leave the room and re-
Enter it to reset the emplacement of the blocks. Take the 2 blocks in the
Room and push them to the right. Place both of the lift there and stand on
The lift too. Press the switch to go up. Grab one of the blocks and push it north. Then push it into the gap
At the right so you can reach the chest with Tomahawk Lance. Go back and
Grab the other block. Push that one into the gap at the left so you can
Reach the other side. There, use the lift to go up again. Check the door
Here and enter the correct pass codes you got from the soldiers. The pass
Code isn't always the same.

The door to the hangar will open, so go in to find the Rheairds. There'll be a boss battle after the cutscene have fun fighting YUAN and BOTTA

You have to use the levatator to the top of the area. Then continue to the door north of you and enter the code. Then you get to fight Yuean and Botta at the same time (take out Yuan fast, make him your #1 priority)