If you want to Kill Bow Guardians With your Sniper then you will require following build.
HeadGear Top : Lkh ( or any which gives you max agi/flee on your server)
Headgear Mid : Binoculars ( or any which gives you max agi/flee/dex/hit/damage/attack on your server)
Headgear Lower : Pipe ( or any which gives you max agi/flee/dex/hit/damage/attack on your server)
Armour : Pantie+Alicel Card
Weapon : +10 Composite Bow +2 Knight of Abyss / Abysmal Knight + 2 Hydra Card
Garment : Undershirt+Whisper Card
Footgear : Black Leather Boots+Male thief bug card/Chonchon Card (best will be Chonchon Card)
Accessory : Vesper core 03 x2
Arrows : Neutral property
Agi : 99
Luk : Remaining (for Lucky Dodge)
Or instead of Luk Take Vit for HP.
How To Kill It
Use Awakening Potion, Attention Concentration , Level 10 Agi, Level 10 Bless , Assumption(optional),Agi/dex Food (optional) and Go Search A Bow Guardian In Thor dun 1 or 3 and Kill him with Ds (try to search with less monsters)
No Need Of Party for doing this. You Can Solo him If you are 90 Level and above.
Thanks For Reading ^_^