I am a solo leveler, so I rely on hp/sp leech all the time. Important things on gear are +ranged/phys dmg, +DEX, +HP/SP. Some ideas:
Upper: +9 Vanargand Helm/+9 Evil Marching Hat
Mid: Robo Eye/Black Devil Mask or situational
Lower: Well-Chewed Pencil
Armor: Glittering Jacket with DEX enchant (cards situational, default card should be Dame of Sentinel)
Garment: +9 FAW with EA enchants/Menblatt card is highest damage.
Boots: +9 Enhanced Variants (Firelock Soldier) are top of the line, cheaper alternates are Vidar's Boots, or Enhanced Variants + Verit/Gold Acidus.
Accessories: RWC Rings with DEX/Atk/Atk enchants will boost dmg the most, carded with Zerom or, if you're loaded, DL Essence Conc3. Sherriff's Badge is also pretty good if RWC Rings are out of reach. If both those options are out of reach, just get as much DEX as you can out of your accessories.
Weapon: Minigun with Hunter Fly cards, or dmg cards if you don't need leech.
For shadow gear, Physical set + DEX armor is best dmg-wise, boots and shield are not very important, I guess appropriate race shield/boots if you can afford them for whatever you're leveling on.
Incubus pet really helps keep your SP going.
Platinum Alter is a good dmg buff to use if you need more power to one-shot your enemies, can use elemental converters with it as well to get around the Holy element bullets. If you still need a damage boost, Gatling Fever stacks with Platinum Alter too, but slows move speed, so it's not ideal if you are walking around a map to level, better for teleporting around.