So here are some Highlights of all the boss kills in vet maelstrom arena on my magicka DK and down at the bottom I will put a link to my tamriel foundry thread in so you can see my gear and bar setups.
Stage 1: twitch.tv/asneakybanana/v/25188016
The most important thing of this boss fight is keeping your wings up. Reflecting the bosses attacks on this round not only stops you from taking damage but the reflected projectiles also do a pretty good amount of damage back to the boss. Youre going to want to kill the first 2 single adds that spawn and then once he teleports after you kill them run to the boss and drop your banner and just burn the boss down to 0 from there.

Stage 2: twitch.tv/asneakybanana/v/25188721
Here you're going to want to make sure you have harness magicka up most the time since it absorbs the bleed from the blades spinning around the room. For the bosses you are going to want to him them with pulsar since that reduces their max health by 10% which is a nice little bonus. Once you get one of the bosses to unshield after you get him to 50% you're going to want to drop ult and attempt to burn that boss to 0. If you dont its still fine just keep working on the rest of the bosses making sure to keep your heals and harness magicka up. Dont use the levers because they cause an additional boss to stand up and walk to the middle which heals them so now not only do you have to deal with 2 bosses but one or both of them is being healed fairly quickly.

Stage 3:twitch.tv/asneakybanana/v/25189307
This is one of the most painful bosses in the entire arena IMO just due to the fact of how hard he hits and the stranglers being buggy as hell. For this boss there arent really any tricks other than making sure you have your hots rolling at all times as well as are frequently swapping back to your resto bar to heal up to full and get igneous shield in there to absorb about half of the next hit. You are going to kill the first 2 set of adds, so the humans and the snake, then push the boss to 35% or so and drop banner and then dps him to 0 ignoring the 3rd set of adds. A lot of the time those adds will go into your banner and just die from the dot.

Stage 4:twitch.tv/asneakybanana/v/25189757
This boss is one of the easiest in the entire arena beside 1 and 8. The boss is pretty much a DPS race. Stand in the green circle under his feet when hes in lightning phase and then during fire phase drop eruption and banner and spam AoE to kill the adds. The trick to fire phase is finding the spot between his pbaoe attack and his ground targetted aoe attack where both wont hit you. Once the adds are dead just burn the boss to 0. You should just make sure to keep up harness magicka during the fire phase just in case you walk out of the safe area and eat a fire or 2.

Stage 5:twitch.tv/asneakybanana/v/25190142
A fairly simple boss. Make sure you dont lose any of the platforms to trolls thats the #1 priority. At 90%, 60%, and 30% he is going to spawn adds. Make sure you keep wings up until the ranged adds kill themselves and mainly just focus on kill the melee adds. For the last set of adds just make sure you keep wings up to the end of the fight and burn boss. It really helps here to run crushing shock in case you are late getting to a troll you just need to hit him with 1 cast and he stops his stomp and gives you time to kill him.

Stage 6:twitch.tv/asneakybanana/v/24990434
One of the more troublesome bosses in the arena if you dont know what you are doing but once you figure out the mechanics its a breeze. The trick here is making sure to always watch out for the web spinners trying to cover up the obelisks and killing the hoarvars near a covered obelisk so that you dont have to use the synergy which takes forever. Another thing make sure you have wings up pretty much the entire fight, even though you arent going to get a lot of DPS out of them it does help out in allowing you to stay on your destro bar for longer and not have to heal a 12k hit every 2 seconds. Other than that just try and get all of the obelisks lit up so that you can stun the boss and make sure you kill the mini spiders quick because after a while the snare becomes brutal. That remains true even when the lurcher is out that you want to make sure to kill the mini spiders quick.

Stage 7:twitch.tv/asneakybanana/v/24990933
Disclaimer: I dont recommend most people do this fight how I do it unless they are very comfortable dodging mechanics and keeping out of range of the bosses melee attacks.
Here I basically just focus boss with crushing shock weave the entire fight. After a little bit he spawns 2 lightning adds. If you keep wings up they will kill themselves quickly enough. However, when the first one dies the boss goes into an enraged scream and you MUST interrupt him or else you will die in 2 ticks. After you interrupt him he will remain enraged for the rest of the fight and his swipes will deal ~17k damage and his stomps ~30k damage so make sure you have both harness magicka and igneous shield up at all times. When the poison lady spawns make sure to kill her quick and not get hit by any of the poison plant's circles. After a while he will spawn the lightning adds again and its just rinse and repeat at that point until he's dead.

Stage 8:twitch.tv/asneakybanana/v/24992526
An extremely sloppy kill here, should have been able to kill him in 2 phases instead it took 4 however this gives you guys a good idea of how to deal with the mechanics in the later phases if you dont have quite as high of dps. Pretty much here you are going to want to burn 2 of the pillars at the start and then kill the fire mage that spawns. After that pull boss to the last pillar and drop eruption and banner on the boss and pillar. Get the pillar down and burn boss as hard as you can. If there are flame atros up you can pretty much ignore them as they hit for next to nothing. Once the boss stands back up kill the 2 flame atros and start working on the pillars again. Kill the flame daedra that spawns and continue to work on adds. If im going for a 2 phase kill I might stall here and try to wait for 2nd set of atros and kill them so that I can get a banner up for the 2nd phase but if you arent worried about a 2 phase kill just kill the 3rd pillar and burn boss as hard as possible. Other than that the fight is simple.

Stage 9:twitch.tv/asneakybanana/v/24993139
Definitely the toughest boss for DK to kill. You need to make sure that you have magma armor up at the start of the fight to deal with the first daedroth. Once you kill the first daedroth focus boss until he reaches 70% ignoring the healer other than for interrupts. Once the boss is at 70% he will spawn a clanfear that you have to kill next to a pad on the ground. Kill the healer and then bring the clanfear to the pad. Once you kill the clanfear go up top and start to work on the first crystal making sure to dodge when you see green sparkly things around you. If the dot up top gets too strong for you I would suggest reapplying shields and then jump down and kill the daedroth using magma armor. Once you drop down just kill the clanfear and head back up top to finish the crystals. For the rock wall part I usually try to face my camera to one side so I know if i dont see the wall I have to start walking backwards and if I see it I have to walk forwards. Once you kill all the crystals you go back down bottom and make sure to interrupt the boss' channel and then immediately start to look for the gold ghost floating in. Position yourself between the boss and the ghost and start to dps the summoner making its way towards the middle. Once you have the ghost and the summoner is dead focus dps on boss keeping your eyes out for another ghost. Once you take the 2nd ghost the boss will teleport away and spawn a 3rd ghost and daedroth. Get the 3rd ghost and use the synergy to give you the boosted dps and time to kill the daedroth. Then focus on the 2nd summoner. From there go back to boss keeping focus on the gold ghosts. He will spawn 3 more ghosts then another daedroth. At that point you should have the boss at low enough health where when you use the synergy you can just focus the boss and he will die. After that congratulations you beat maelstrom on DK.

Build: tamrielfoundry.com/topic/the-magnus-destro-pve-dps-build/
Here you can see my gear setup (using julianos not twice born) as well as my skill bar setup and mundus stone selection. If you have any questions about gear and skills for the arena or just pve in general you can ask there or here and I will try my best to answer everything.

Best of luck to everyone attempting this and dont get discouraged by a couple of deaths