The following is a basic skill build for an Explorer who wants to grow up and become a sniper!
The guide only takes you to level 43 just after you become a sniper. After that you are grown up enough to walk your own path and invest in skills that most fit you.
If you choose to follow this example, by level 43 you will master two important skills: Concentration, which helps to avoid missing shots and Wings of Wind, which makes you move as fast as possible on your own strength, without extra items. Some skills are good to invest in at the beginning, but as you become stronger, you gain access to skills that produce more damage. You might want to invest in the later because after all a Sniper is all about causing damage!

Important skills for a Sniper wannabe
Concentration
Increase range up to 3m and accuracy rate up to 212. This influences the success of the attacks. This skill should be maxed.

Rifle Mastery
Add long range striking power up to 430 when using a shotgun. This influences the overall power of attacks with a shotgun. This skill should be maxed.

Burst Shot
Add damage up to 1136 per shot and disable/stun target for 5 sec.

Aimed Shot:
Add damage up to 1072 per shot and decrease target’s moving speed -60% for 8 sec.

Quick Shot
Add damage up to 960 per shot. This skill can be used by shotgun and dual guns.

Hit Chain
Increase accuracy rate by 91 for 10 minutes. This skill can be used on other players also and influences the success of the attacks. Try to max this skill as you grow.

Poison Bullet Shot
Add damage up to 623 per shot and poison target by 241 points every 3sec. This effect on the target will last up to 21 sec.

Shooting Nova
Add damage up to 3209 per shot.

Sniping
Add damage, ignoring defense of target and attack very strongly.
This skill will probably be you favorite! there are no known players who have maxed this one out because you have to be lvl 90 to master this with every new level you invest in this skill it will grow with adding 100-300 more damage than the level before. At lvl 1 of this skill it adds 117 to damage, at lvl 9 of this skill it adds 1666 to damage.

Wings of Wind
Increase movement speed by 40% for 15 minutes. This skill makes Explorers faster than other players.

Raise Agility
Increase Evasion rate. Try to max this skill if you have more points in dex.

Optional Sniper skills
Aerial Wings
Increase avoidance rate of party members up to 42 for 10 minutes. This will make a Sniper popular in Florensia!

Balance Up
Raise resistance against critical damage and stun up to 80% for 10 minutes.

Frame Splash
Damage targets in the area in front of you using a shotgun up to 1184 per shot.

Invest minimum in the following skills
Fishing
Some players make a different character just for fishing. You can choose to do that or invest some skill points in your main character.

Skills not worth investing in
Net Launch
Makes target in certain range not to move for a certain time.

Calm
Calm monster, and decrease menacing level.
If you want to use Aerial Wings, you will have to invest one point in this skill.

Pierce Shot
Pierce your target and targets behind in a straight line by using a sharp bullet. This only works if the targets are almost in a strait line or very close to each other.

IMPORTANT: Avoid all the skills of the dual guns and swords. Concentrate on the shotgun skills to produce more damage. U can use the dual guns if you like to try them with the quick shot skill. Any more points would be a waste as your damage is done by the rifle!

Of course if you want to experiment with other skills you can always buy a skill reset stone ^^ or invest in Fishing instead of creating another character for that like in this example.