For the first clear, here is a couple of things I would consider.

First goal is to reliably be able to get upstairs. It is imperative that you dont leave a healer downstairs, not an issue to leave a guard. Sounds like you have that for the most part. I think the first stage is certainly ultimate worthy if you have one. You really shouldnt need to grab any sigil here, but the speed sigil to go upstairs is nice. Keep in mind that the speed sigil also boosts all primary stat regen.

If you can kill the crystals in one go around, great, but it shouldnt be required. Speed sigil and tri pots are your friend up there. Running out of stamina is likely a wipe. If you need to jump down, do it proactively. In other words, buff and shield before you jump because you will have a guard in your face. My trick for the crystals on classes I am not super comfortable with:

-Grab speed sigil and kill healer, and port to top.
-Drop every ground/ST dot you have on the crystal closest to you.
-Use your spammables to attack the second crystal, I usually go clockwise to the next crystal.
-You should be able to get 2 down that way by the time the first shield spawns, or at the very least, get two into execute.
-Once shield spawns , move Counter clockwise (opposite the direction the shield moves) to get behind it.
-Once safe, lay down DoTs and take out the third crystal during the shield phase.
-Then you should be able to execute anything that is left.
-At the end of the second shield phase, I advise buffing and jumping down, unless you are really close. With practice, you wont even see the second shield.

On the last wave, assuming you killed the healer, there should not be a guard to deal with. I might be wrong on this mechanic (been a while), but I believe letting the healer live in the first phase is what lets the guard stay up after the explosion from getting all 3 crystals. If you do need to drop down, I advise you kill everything down there.

When you kill the last crystal, move directly towards the shield sigil. Also a good idea to pop your own shield just before you kill the last crystal. The shield sigil is important because it will reflect his channel attack. Once you get the shield sigil, grab the power sigil, the entire time, your head is on a swivel for golden ghosts. Always go for the shield sigil first, but definitely deviate to grab a ghost that spawns before the power sigil.

Once you have both, nuke that mother. You should have an ultimate, also as a templar, use radiant glory so your execute gives you a heal. (sorry, you said magden, not magplar). If you get all three ghosts, wait for the guard to spawn before using them. Once you explode the room, you have a choice to make. If the boss is close, finish him. If not, kill the guard first, rinse and repeat. With really low DPS, I would advise you find a spot for crushing shock so you can interrupt if your shield sigil wears off. The sigil will get you through one round of ghosts (plenty of time with good DPS), but you will have to manually interrupt after that.

With a little practice, you will simply jump off, bash the boss to interrupt and nuke him where he stands. With good DPS you wont see a guard or more than 1-2 gold ghosts, but that is not they way to learn it.

You can actually kill this boss without killing a guard. Nuke to 70% before he ports, do cystals in one go, execute before guard spawns. You will get three ghosts before the final phase guard spawns, assuming you dont have one up already.

Edit: Also, trap will halt a summoner in there tracks to by you some time in the final phase if that is an issue. If you dont kill the boss by the first spectral explosion, you need to take out the first summoner. If you have reasonable DPS they can be completely ignored.