You only need Winter in the LAST mission, not all of them

and thats only if an ele in your group refuses to be anything but fire.
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Anyway, yea, there are standard builds everyone and their momma runs in Pve cant really solve it, just find a group that lets you in.
SS is though, Spiteful Spirit, capped in the SOuthern Shiverpeaks, which you havent been too yet, but they STILL expect you to have it in the jungle, because everyone just gets run to Southern Shiverpeaks when they start a character anyway.
Spiteful Spirit is, too most, the best PvE Necro skill (or best pve skill in general) in the game.
15e, 2 cast, 10 recharge
For 8-18 seconds, whenever target foe attacks or uses a skill, Spiteful Spirit deals 5-29 shadow damage to that foe and all adjacent foes.
Ok Cost, Ok Cast, Awesome Recharge, Awesome Duration, Doesn't cause the AoE scatter, and does the most dmg of ANY CLASS as long as the enemies are grouped together, like they always are in PvE.
Thats why it is so popular for a necro to bring. YOu are /me so they expect you to be doing Arcane Echo and SS together, putting Spiteful on 2-3 of the enemies in the group, basically making them drop in 2 secs...
SS/SV is also a popular duo farming build for the favor locations. You get a 55 monk to tank (55 means they have 55 health, and prot spirit on, so they only take 5 dmg a hit - so they can tank alot of enemies), and the necro does the dmg by putting SS on a couple of the monsters. SV is a mesmer skill Sympathic Visage, it makes monsters attacking the 55 lose all adrenaline and 3 energy, which helps ALOT with a few certain monsters that yuou dont want to have energy.