The OP never mentioned solo. Why did you?
With extensive testing of the Tempest, and the most precise explanation you got is "awesome"? Interesting.
Let's do this dance again. Who doesn't love a pissing contest between strangers?
Here is what I wrote to an earlier poster wanting to ****** in cross with me. You are allowed to disagree -- but actually back it up witout assuming, okay? 
Here is the key movement about Tempest and Assassin. Testes through and through.
- 2 different playstyles. Assassin builds on burst, stealth and damage. Tempest build on control, survivability and fun effects.
* Tempest abilities consist of 3 flask and a focus ability. these are: Flask of Fire, Flask of Frost and Flask of Lightning. The focus ability is calledThousand Cuts.
* An assassin gets 3 abilities and a focus ability. These are Hidden Blade, Knockout bomb and mark of death. The focus ability is called Cloak of Shadows.
First off: You do not judge a class based off of one ability. You don't. Doesn't matter if it is broken, OP or otherwise amazing. Hence we do not judge Assassin versus Tempest on Mark of Death vs Thousand Cuts. That would be like comparing a shotguns reload speed to a pistol. They both can, and will, kill any enemy in the game as they finish -- but Thousand Cuts is both on a long CD(You need to gain three stacks of focus!) and ends faster than Mark of Death.
That means we can isolate the difference into 2 cases: As a whole class, and as a specialization. Let's do so. Let's compare the specialization up to each other, and see what sort of funniness(is that a word?) we get!
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Tempest
Active:
- Flask of Frost - 85% damage resistance, 5 seconds, freeze for 1 second, 32 second CD.
* Area of use: As a survial tool, or a tool to freeze AoE. Mediocre damage ability.
- Flask of Fire - No cost or CD on abilities, 5 seconds, 32 second CD.
* Are of use: Able to spam attacks. As an archer this equals 3 Long shots or 2-3 full Draw. As a melee this equal 4-5 Twin Fangs or Deathblows. Good damage ability.
- Flask of Lightning - 2 effects, depending on whether you control the character or not. Either a huge attack speed modifier, or a slow down. 7 seconds, 32 second CD. Good survival, decent damage if you are not controlling. In this time you got enough time to do 1 combo. Stamina is a ******!
Passives:
- Flaskmaster - 25% to not use a flask. Random -- but decent. 3 dexterity.
- Fury of the storm - Damage bonus on low stamina. Good within Flask of lighting - rather useless with Flask of Fire unless you plan it. 3 constitution (lol)
- Ride the Storm - 3 second if you active a flask right after another. If you plan it out, this means you can get an extra 2 auto attacks off in FoL, another Longshot or full drawn in FoFi or live longer in FoFo. Decent all around. 3 dexterity.
- Killer's Alchemy - Gains damage on using an elixir of potion. Very strong combo, as it stacks. Using a potion + an elixir gives 30% damage for 10 seconds. 3 consitution (lol)
Focus ability:
Thousand Cuts - 38 hits. Each one does 300% weapon damage. In itself it is enough to drop a high dragon to half health.
Assassin
Active:
- Hidden Blade - 6 hits, 65 stamina, 32 second CD, 300% weapon damage, ranged. High damage ranged attack.
- Knockout Bomb - 10 seconds, 35 stamina, 20 second CD. AOE CC. In itself it's just a sleep CC.
- Mark of Death - 8 seconds, 10 stamina, 32 second, 20% armor reduction. Damage taken in these 8 seconds will be added as it expires. This can also crit.
Passive:
- Throatcutter - 2% damage for each 10% missing health. Mediocre.
- I was never here - Killing an enemy reduces the CD on stealth. Strong on mobbing, useless on bosses/dragons.
- Knife in the Shadows - Auto crit from stealth (!!). Very, very, good.
- Gaps in the Armor - 25% passive armor penetration. Frees up stat points that would otherwise need to go into armor penetration.
Focus:
Cloak of Shadows - Your team goes into stealth for 9 seconds. They also gain the stealth buff, meaning 50% damage increase as their first attack.
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Now. What does all this mean? Simple. A Tempest is limited in his/her gearing choices. For optimal damage you need to reach close to 70%-100% crit chance. That takes a lot of points. An assassin, on the other hand, does not need this. His crit is already 100% out of stealth, which is all he needs. This means every single point a tempest get into crit, the assassin gets into Flanking Damage or Crit Damage. Obviously some crit is wanted, due to Mark of Deaths ability to crit.
If we look at synergy between trees, as a class on its own. There is a few obvious thing to notice:
1) Pincushion is the reason Thousand Cut can kill a High Dragon. 38 hits, each hit increases the damage by 5%. That's 190% damage increase by the end.
2) Cheap Shot increase armor penetration even more. In Mark of Death, you got a passive near 100% armor reduction on the target. Whilst a Tempest gets.. none?
3) An assassin can utilize Knockout Bomb with Mercy Killing for some absurd damage. From stealth it means 1-shot to any non-tank enemies. Even if they are tanks, they die within a combo.
4) A tempest can utilize Shadow Strike within to have near 100% uptime on flasks. The damage is mediocre -- but it's there.
5) Flask of Lightning + Mercy Killing + Knockout Powder is a valid combination. Most of the time it will be auto attacks.
6) Flask of Frost --> Flask of fire + shattering is a valid combination. But you can't do more than a few shatters.
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At a min/max perspective an assassin outclasses a tempest in damage. Yes, yes, I hear you "But hurrr durr dragon died in 1 focus ability :<". Yes. It did. It also takes you.. 5 minutes to get 3 focus? Which means it is only used if you use it rarely. So, let's say you have killed no High Dragons. I tell you to race through them as fast as possible. Who wins? The guy who spends 5 minute fighting enemies to get focus, or the guy who kills it in 8 seconds?
Why is this, you might ask? - First off. Assassin don't need to build for crit. That's the big clue. Crit takes a lot of stat from a Rogue. This could be used for flanking, which is huge, or crit damage. which is also huge. The theoretical damage is miles beyond what a Tempest can hope to achieve. That said, a tempest is a different playstyle altogether. A team player. FoFo makes for good "OH ****!" times, FoFi makes for good damage and FoL makes for good positioning or poison-spreading.