This is a guide for the Sea of Moving Ice skirmish Svardborg (referred to as "nSVA" for the normal version, and "eSVA" or "mSVA" for the Master version).
To enter nSVA, you must have 28% everfrost resistance (only needed for entry, not required once inside), 2800 item level and have the instance unlocked in the Storm King's Thunder campaign.
To enter mSVA, you must have the normal requirements, 3100 item level and you must have completed Fangbreaker's Island.
The only difference in difficulty between the 2 levels of the skirmish beyond the requirements above is that in the master version, you can not rejoin the fight if you die. In the normal version, you can re-enter any time.
As far as loot goes, Master has a chance to drop Legendary Marks rarely, but drops Rare and Epic Marks consistently. Normal drops Blue Marks only. Both have a chance to drop new epic. Legendary rings can drop in Master.
The marks and Rp that is dropped specifically relate to the new weapons that need to be restored and leveled. They can be leveled with normal RP, but NOT normal marks. It takes 5 of each type of mark to upgrade each weapon (so you will ultimately need 10 of each blue mark, 10 of each epic mark and 0 of each legendary mark, to fully upgrade the weapons.
Chests: There are 3 chests available when you win, each tied to a Sea of Moving Ice reputation. If you have 1 point of rep, you can open the corresponding chest. Currently, you can peek and not take, but this will likely go away soon. You may also use legendary keys if you don't have the reputation.
The first phase involves defeating the boss's army. Kill everything you can and pay attention to the attacks targeting you. This phase is relatively easy, but again it is a timed instance so the faster you can clear this the better.
The best way to do this is to fight on the rug and have players marked with the Ice AOE to move it away from the areas where the parties will fight. So in the first phase as long as you keep it away from the area near the stairs there shouldn't be any ice AOEs on the boss when you start phase two. Fighting on the rug means that all the mobs come to you and your healers have a much easier time dealing with hypothermia and players are freed from permafrost more easily. It also means that proximity buffs like Weapons of Light, paladin auras etc. are hitting your DPS.
Once the army is defeated, the boss will join the frey. IMMEDIATELY hide behind the pillars on the left and right side of the instance (when looking at the boss's steps). If you do not, you will die. Stay behind the pillars till the attack clears, and then engage the boss.
During this fight, the boss will call on the ring of winter, and Runes will appear on the floor that have swirls going toward him. Ranged characters should stand on these runes to lessen the impact of the impending attack. The more runes are uncovered, the more damage is done. Once the attack is done, go back to attacking as normal.
The damage reduction from runes isn't linear so you only need to cover three runes in order to survive. If you have less than three players on runes the players standing on the runes will also take increased damage that will likely one shot them.
I would like to add that the tanks should move the boss a couple steps out from the steps and then be between the boss and the steps during the boss fights. DPS and clerics should be on his back side facing the steps. This will help minimize the damage done to the main DPS units. If this is not what you want to do, at least get tanks on one side and dps/clerics on the other.
The only thing I think you missed is the root that the boss has for tanks. I'm not too clear on the mechanics but it can one hit tanks and that's part of why you have to two tanks to split the effect. AA can mitigate this problem so timing it for when you see arrows on the tanks can make your tanks' job a lot easier. Although honestly AA can let you ignore virtually all the mechanics aside from the Ice AOE because it's broken and OP.
At 75%, 50% and 25%, 2 Manticores will be called to the center of the instance to fight you. Best bet is to focus down one of them at a time. They seem to have a knockdown attack but overall, the biggest threat during this phase is the close quarters (you are locked to the middle area), and the boss's attacks. This is timed separately and they will leave at the end of the timer or when they die. If they both die, I believe they both respawn the next time with little health (confirm?)
- (add from @mercedesman)if two tanks are running, decide up front which Manticore you will take and focus on that one. Try to keep them in the center of the area as it is easier for the dps players to get to them.
After the 3rd Manticore phase, another giant will come and prompt you to enter the middle of the instance for protection from a major attack. If you aren't in the circle, you die (even if you are dead already and in the starting area. She then sends you on your way to finish it.