Mercenary
Gemling LegionnaireThis is a Chinese build archetype first showcased here by 可布和常常诶, and then popularized on Witchhunter by Lazy Exile - who is an amazing build making and content creator. If you're interested in their variations please go check them out!
This build relies on 
Frostbolt with Breachlord's Amalgam to INFINITELY Proliferate with 
Mirror of Refraction. Each Frostbolt that hits the Mirror of Refraction will split into 10-12 projectiles and then return to you via Breachlord's Amalgam and if you stack enough CDR you will be spawning mirrors at like 2-3 per second which means you can ramp forever and be able to just have thousands of Frostbolts flying around your screen at any moment. There becomes so many that the game gives up and doesn't even show their animation making this one of the builds that has the MOST visual clarity in the game.
Our Defences are phenomenal, our Damage can quite literally scale to infinity, the only stopgap is mobility, which can scale infinitely as well, but at a higher investment than normal. This is because we need to balance our mobility with the speed of our Frostbolt and the CDR of our Mirror of Refraction.
The only important pieces for this build are 
Atziri's Rule for Mirror of Refraction and a high quiver effect 
Widowhail and well rolled 
Drillneck (high crit and damage). Everything else is completely up to your ingenuity to balance out. Because Frostbolt snap shots Drillneck, and we pierce infinitely we essentially get max crit chance and like 100's if not thousands of increased damage from our quiver. As a result the rest of the gear and tree can focus on scaling defenses or utility.
DEFENCES: We can GIGA invest into defenses since our weapon setup gives up so much damage. Because Gemling is OP we can get insane amounts of %inc Armour from 
Arctic Armour alt quality and we can also invest heavily into Evasion and ES. These layers of defense all work multiplicatively with each other to create a MONSTER TANK with only 8.5K ES which Can be scaled on all vectors pretty insanely
FAQ:
My Chain Keeps Breaking:
If you notice that your chain is constantly breaking this is either because you're in a reduced CDR map OR that your Proj speed is too high. In which case reduce proj speed on Jewels and remove Projectile Acceleration III from your Frostbolt
How Do I Increase Mobility?
The one downside of this build is that cranking up the mobility is a 3 part equation. Normally, that faster you run or dodgeroll the faster you are. But for us to increase mobility we need to increase 3 things simultaneously.
1. Our Movement Speed
2. Projectile Speed
3. Mirror of Refraction Spawn Interval
1. You Can increase MS From Boots + tree + Runes/Dodge Roll distance (
Cast on Dodge quality)
2. You Can Increase Proj Speed With Supports Like Projectile Acceleration III, Jewels With Proj Speed, Tree Nodes
3. You Can Increase Mirror of Refraction Spawn Interval Through + All Levels NOT Spell Skill Levels and through CDR on Jewels, Tree, Gear, and Supports (Cooldown Recovery II)
Grim Pillars Vs. Frost Wall
Grim Pillars and 
Frost Wall both serve the same purpose
1. provide ice crystals for Verglas support to give us gain as cold damage
2. Provide an "enemy" for our Frostbolt to pierce for Drillneck snapshotting
Grim Pillars provides way more pierces which is way more damage, but is super annoying for traveling since the walls block mobility. So if you're trying to go fast go Frost Wall and if you're trying to kill tanky mobs like wisp strongboxes or Juiced Expedition then go Grim Pillars
