Job 2: Crossbowman or Hunter

(Cross)Bow Mastery
This is a necessary skill. Mastery increases the minimum damage of your attacks, which greatly increases damage consistency and thus your training speed. 20 Mastery does the exact same thing as 19 Mastery in terms of damage, so it is pretty much worthless to get 20 Mastery.
Final Attack
There is a great amount of debate surrounding this skill. Refer to the FA vs FA-less discussion. In essence, when maxed, you shoot another arrow 60% of the time with the power of 250% (350% with critical) when using Arrow Bomb, Iron Arrow, AB, or DS. More commonly used with AB builds than DS build. Also more common for xbowmen to use than hunters. Requires 3 points in Mastery
(Cross)Bow Booster
This is a necessary skill; however, it does not become more effective with additional points spent into this skill. The timer grows and the HP/MP cost decreases when points are added, and thus level 1 Booster has basically the same effect as level 20 Booster. The difference in timer and HP/MP cost between level 20 and level 1, however, is quite significant. Refer to the speed section for more information. Requires 5 points in (X)Bow Mastery.
Soul Arrow
This is yet another necessary timer skill. Like Bow Booster, it does not become more effective with additional points spent. This skill gives you an infinite amount of arrows for the duration of the timer. When maxed, provides 10 minutes of Soul Arrows. Requires 5 points in (X)Bow Booster.
Power Knock-Back
This is the first melee skill archers get. You may notice that when you bow-whack low leveled enemies, they sometimes slide across the screen. Power Knock-Back can do this to up to six enemies when maxed. It also adds to the (X)Bow's percent chance to cause a Knockback, as well as claiming to do +200% whack damage.

It is somewhat a mysterious skill that appears to factor in level or KB resistance from the enemy. The attack damage equation is also unknown, as it has extremes of highs and lows. You can also use it to kill mages by knocking Golems or other large creatures into them. Just keep in mind, Nependeaths will never slide from any amount of PKB, no matter how hard you try.

Most people think that PKB adds the % KB rate weapon and the skill. Aka, a lvl 90 Metus will give a 100% KB rate. Unfortunately, this has been proven wrong with several different bows, and a large amount of tests. Leaving us with the conclusion that no one really knows how this skill works. Current tests point at having a 60% "KnockBack" cap, but even this is uncertain.

The test was simple: At level 40, keep using PKB against Stone Golems until they jump back. Easily, you can see that it is less than 5% of the time when Golems get KBed, or even flinch. Even with Max PKB, you don't get the theoretical 70% or 80% Knockback. Some variables that may effect the PKB level are monster's level, an unknown monster "size" variable, and maybe even how many enemies you hit. But all are just theories and are not even tested.

PKB takes advantage of a glitch or a "feature" that afflicts crossbows. Often, when an enemy is close, the crossbow will shoot, but because of slowness, the enemy will advance upon you and the crossbow will bash the enemy instead of shooting. While on the surface that would appear to be a disadvantage compared to a bow that does not suffer that problem, it is in fact a feature that allows a combo. If the Crossbowman anticipates bashing the monster, instead of continually shooting, he can activate PKB and cause it to chain in a combo after the previous shot, forcing the enemy back far enough to be shot again, or knocking them back, assuming their KB number is lower than the damage done. The point is that if you are going to be forced to bash the enemy against your will, you increase your chances of knocking them back by activating the PKB skill. If you don't do it, you only get the chance for KB inherent to the crossbow, but if you use the PKB skill you get the KB% from the crossbow plus the unknown bonus you get from PKB.http://strategywiki.org/wiki/MapleStory/Bowman_Guide/Skills