Me too, I have the same problem (also being stamina nb). The solution is described above - try to finish the crystal phase in two steps. In general, here is what I am doing through the fight with the last boss:

Phase 1: the goal is to dps the boss BEFORE crematorial guard is spawn (you need around 30k dps for that). Having this much of a burst dps on a stamina nb is not that difficult - rearming trap, endless hail, caltrops, leeching strikes, relentless focus before the boss and then: incapacitating strike, rending slashes, and then spam suprise attacks on a boss himself. Do not forget to block his skull attack in the meantime though. It is also good if you manage to kill the healer before going up

Phase 2: use the portal, put poison arrow + rending slashes dots on the first crystal, vigor and roll dodge (usually, left), then put poison arrow and rending slashes on the second crystal. I also try to put endless hail on one of the crystals, depending on timings. Then, I return back to the first crystal, re-apply both dots, then vigor and keep moving right, and apply the same dots to the third crystal. At this point, the wall is appearing, and you re-apply dots to the closest crystal, whatever it is, usually the first one. After this - it is up to you, you will have two roll dodges, ideally if you manage to put 2 more rounds of dots/endless hail, but at any point - when you feel you do not have stamina anymore (it comes with experience to "feel" the moment), usually it happens somewhere around second wall. Then you walk down and pick up defense sigil, and optionally - the health one. Having both sigils makes fight with a crematorial guard relatively easy - as far as you avoid red circles from the boss. Then, before going back, you pick up the restoration sigil and get back on top. Now, you should not have difficulties finishing the phase, but it is also important to finish the phase with relatively high stamina (that is why we picked up the recovery sigil to let stamina recover while we are up).

Phase 3: You jump down, bash the boss, and apply dots/endless hail/caltrops on the boss, and pick up golden spirits to "stop-the-world". You ignore everything else but the boss (you re-apply dots when he jumps). I usually also pick the axe sigil at any convenient point - but it does not matter much when, as either your dots will give more damage, or your execute phase if you will pick it up for the "stop-the-world". Either will work.

This tactics allows me more or less consistently (I still make mistakes here and there) deal with the last boss.

One tactic that seems to be safer is to try to damage all the crystals down to about 30%, then come down, kill the healer and crematorial guard, get up again, execute crystals fast. The reason to do it like this is because the speed of the protective wall seems to depend on the number of intact crystals, not just the time you've been up. Also it always spawns near the one with the highest health, so if you are damaging that one it will be easier to reach it and hide. Once the crystals are destroyed and the boss throws you down, get the defensive sigil and let the boss channel his attacks and keep DoTs on him as the sigil will reflect them back, kill the summoners and healers so they don't summon the bone colossus and heal the boss. Grab the yellow ghosts until you got the synergy loaded. Use the stun synergy just as the crematorial guard spawns near the boss to stun all, then cast ballista on boss and execute before stun ends. You will have enough ultimate build up till that point to cast it since all enemies are Daedra so you may be able to cast Ballista 2x or cast a Ballista and Dawnbreaker or Incapacitating strike.

The upstairs is all about dps. I drop endless hail and caltrops on the crystal, poison injection and rearming trap, then I spam a few light attacks and surprise attack and blood craze and it should be on the excecute range then its just a few Killers blade. Incap. is a great ulti here and dont forget to use your weapon power potions and relentless focus.

For example:
https://www.youtube.com/watch?v=YX7PrD9eSU0

This is tought from the earlier patch but the idea at the upstairs is almost the same.