Hello puddings
My name is secretmoon and  I show you how make  Harley Quinn as aggressive support with amazing healing, very good  AP dmg and 30 - 40 % cool down. If you want use this guide for GD you must buy first Nil Ring

Items
A) You always start with  [[Abin Sur's Lantern (2)]], i know for start you want something with hp or dmg, but we don’t make Harley Quinn  ad, and [[Abin Sur's Lantern (2)]] give you will, will regen and of course you can start get benefits from killing, assist.
B) Second  you need Atlantean Royal Seal (4), I chose it because this give you hp and most important you can slow enemy use skills. This item can save you live or you can easier kill enemy, remember you are marksmen with low speed, that why this is important item.

C) [[Abin Sur's Lantern (3)]], for more will and cool down you get 20% cdr if you have full stack 
D) Cosmic Staff (3) for cdr, dmg, wounding and disintegration which give you overtime dmg important for killng and assist
E) Eye Of Ekron (4) which give you more will to heal, more slow for  W and dmg for Q, and + 10 move speed now you rally good runner for catch enemy and run away if you need.
F)Next  you buy  The Starheart (3) this item give you big dmg boost,

G)Last item is choice: if you want more damge vs heavy team (tank,bruiser) The Book Of Eternity (2) is good choice, [[Fatality's Energy Lance (4)]] give you big dmg and cdr. My favorite is Cosmic Belt (3) give you hp some ap and power penetration is elastic iteam you can use vs big dmg team, or light team witch less power armor (i know The Book Of Eternity (2) give you 35% armor pentration but blaster, dmg and sometime supports have 37- 60 power armor that mean you have more penetration vs light enemy or only little les than The Book Of Eternity (2)), Psi-Scimitar (3) give you ad, ap and Skilled attack which is nice boost for yuor basic attacks,

H) [[Abin Sur's Lantern (4)]].

Modification

Most important is
S.T.A.R.   Empowered Cosmic Staff Disintegration: Dealing damage from skills applies an effect dealing 2% of the target's current health per second for 4 seconds. If the target is hit when below 50% Health, the effect applied deals damage based on missing Health. (Tier 4)
S.T.A.R.   Empowered Eye Of EkronWill Force 2: Grants Power Damage equal to 4% Bonus Will (Tier 4)

Choice:
S.T.A.R.  Accelerated Abin Sur's Lantern Will Harvest: 2 stacks from kills/assists (T ier 2), faster you get more will and 10% cd
S.T.A.R.  Enhanced Fatality's Energy Lance +15 Power Damage (Tier 3)
S.T.A.R.  Enhanced The Book Of Eternity +15 Power Damage (Tier 1)
S.T.A.R.  Empowered Psi-Scimitar Skilled attack 2: After skill use, your next attack deals 50 + 50% Power Damage or 150% base Attack Damage. (Tier 3)
S.T.A.R.  Enhanced Cosmic Belt  +75 hp
Augments
Ap and Ap penetration and one augments for move speed

Important you need 30% cdr because you can use Q 3 sec + Atlantean Royal Seal (4), you get survival skill, witch enamy have problem to catch you and run from you