All in all, this class is pretty balanced. Can do some good damage with their mixed barrage of normal attacks and skills, and can do some impressive damage if they land crits. The scout tree, however, is incredibly linear. While there are a decent enough amount of available skills, most are kind of.. useless.
First of, we have aimed shot, double arrow and triple arrow. Double arrow and triple arrow have a somewhat OK amount of skill power, but the skill just takes so strenuously long to cast that they're not worth it. Aimed shot takes even longer to cast and has even less damage, this skill is by far the most useless thing I've ever seen. As aimed shot and double arrow are hawker skills, we'll leave those aside, but the triple arrow skill needs some love. If at all possible, increase the casting speed, or make it so that the three arrows are shot all at once, not one after another. It's mainly this that causes it to take so long to cast.
Another thing I would like to point out is the impact arrow skill. This skill is impressive for its massive skill power ratio, and with it comes a long cooldown (slightly too long in my opinion, but still fine). However, I don't really understand the point of the PvP impact arrow skill. Stuns are nice and all, but with a 30% success chance it's merely a gamble and not worth it, given its long cooldown. Either increase the stun chance to 40%, but this leaves another problem. With how fast paced combat is, stuns are incredibly overpowering and I'd rather not see so many stuns in the game.
It seems impact arrow always forces a crit, hence its long cooldown and casting time. I'll still say that 30% is too low of a gamble to be worth investing in though, up it to 40% please.
Also, the casting time is as strenuously long as aimed shot/double arrow/triple arrow. Is the skill power/forced crit really worth a casting time of four times as long as skills like phoenix arrow?
Next up, we have fire, ice, wind and poison arrow. While they cast relatively fast, the skill power is so low that you can do just as much or even more by using auto attacks. I'm not sure what you're planning to do with the elemental system, but with how low the skill powers are, and how unreliable the system seems, it doesn't seem like anyone would want to use them, even when it is in place. The poison arrow skill has that poison effect, but in fact no one really wants to use it. The reason being, the burn effect of phoenix arrow by far trumps the poison, especially with how fast paced combat is people will choose a short term high damage DoT over a slow but long one. Not to mention cause of the ability to recast, in 20 seconds one can fire three phoenix arrows, and have a higher accumulated DoT than the poison did. So no one really wants to use this skill either, as DoT's can't override eachother. Here's a few suggestion that may make these skills more desirable:
1) The first possibility would be to increase the base skill power on all four of them by 20%. This would put them at 140% skill power, which still isn't the greatest, but it makes them slightly more reliable and more useful.
2) Move fire, ice and wind arrow to the scout tree, increase their skill power by 40%. This would put them at 160% skill power and is an alternate suggestion similar to the first one. A lot more reliable than 120%, but I'm not sure how it pays out. But it's one of the possibilities.
3) Leave the fire, ice and wind arrow as they are, boost poison arrow to 140% skill power. However, add a new skill, namely the following:
Elemental mana arrow
Skill power: 100%+10%*SkillLvl +200+10*SkillLvl.
Magic targeted skill. Element: Sorcery
Cooldown: 8 seconds. Cast cost: 25% of max MP.
Skill requirements: Fire arrow (5), Ice arrow (5), Wind arrow (5), Poison arrow (5).
This would be the ultimate elemental skill for the scout, combining the four elements you know to create a substance similar to pure mana. The thing that defines this skill is that is a fast casting (same animation as the other element arrows) magic attack. Magic attacks can be incredibly devastating depending on the target, and could mean quite a lot. As such, this skill has a drawback, which is its incredible mana cost. If you aren't careful and use this skill too often, you may find yourself in MP troubles. I'm not sure how it would play out in real combat, as this is purely theorethical, and as such some things may need to be altered.