Whenever you are up top you have a DoT on you, the damage can really add up over time so you don't have to destroy all three crystals in one trip. You can roll off the top into the lava after the second crystal is down, you'll have to kill the Crematorial Guard (Ulti if you must) but you will be more prepared and in control. This is very important, being calm and in control.
When you go back upstairs the DoT resets, you can light attack the last crystal down to build up one more Ulti without taking much damage. When the last crystal explodes and you go back down, get the shield sigil first, it will reflect the Boss's skull attack (don't interrupt it). If you are a lower DPS build you will have to face one last Crematorial Guard and a summoner will appear, use your Ulti on them and you're home free. Keep an eye out for golden ghosts and you should get a Spectral Explosion which will seal the fate of Voriak Solkyn. Keep calm, you'll get it.
BTW the Crematorial Guard is vulnerable to lightning damage.
Ya if you cant get all 3 crystals killed, its sometimes best to control the fight and jump down yourself rather than getting knocked down into lava and CG will kill u on spot. If u have to jump down in the middle and fight the CR there, u get lava attacks on the edges. Drop ulti on him, and other aoe also (spears,wall), keep shield on yourself and just rotate around him. He's fire barely hits you then and he will die in aoe and ult itself.
Also make sure you are not vamp stage 4.
You've gotten some really great advice. Know that if you made it this far you can surely beat him, it took most of us many, many, many hours, days, and weeks to do it. But when you complete it, it will feel amazing. All your practicing will pay off. Your second run will probably be under three hours and as a templar would likely get you on the leaderboards just for a completion (at least for xbox).
Step 1 - burn the boss, if fast you can go up without spawning a CR guard, if not kill guard, healer then go up
Step 2 - Burn first crystal, try learning how many skills you can apply before he throws a skull, its something like 4 light attacks + skills then dodge roll, then keep attacking, moving constantly to avoid the lava bombs. If good dps this crystal will be destroyed
Step 3 - Look for the wall, if it is far away head to the right crystal, lay dots, attack, then wait for crystal to meet you and then follow it. If the wall is near you follow it to the crystal on your left. Lay dots...wait for boss to send out AEO and breaks wall
Step 4 - DOTs on crystal + attack and watch boss for skull attack..dodge roll.
Step 5 - Now is decision point....you can intentionally drop down and take on the CG, reason for this is it is controlled and it will restart the high damage dot. Or finish off 2nf crystal and then head to 3rd crystal
Step 6 - Look for next wall and get dots on 3rd crystal, keep healing as the dot will be getting strong. Burn this crystal then drop down. If there was a CG up it will be destroyed.
Step 7 - GET THE SHIELD SIGIL (as greenmachine points out). It is so helpful it should be your priority as you fall...just keep looking for it and run to it, don't worry about anything else. Do not interrupt boss his attacks will be reflected back to him. Put DOTS on boss and look for GOLD ghost. Grab power sigil, drop ulti ...look for 2nd ghost... keep attacking if you get all three ghosts this should happen right as CG spawns.
Step 8 pop the synergy and burn the boss. If you have good DPS you can grab first gold ghost and just burn the boss down. But I would not try that for your first time....think if I get all 3 ghosts I should win.
One other piece of advice is consider using tri-stat food, since this phase is really helped by blocking, dodge rolling, and sprinting. The extra stam can really help. Also consider using tri-stat potions, or health+magica+crit potions.
Good luck you can do it.
iv been circling it while trying to kill it but it gets me on the last percent of its health, but im gonna try and work on getting all 3 crystals down as that seems to be the easier option since i can get 2 down and 3rd almost i just gotta stop making silly mistakes when i get to the last one and be a bit more observant