The game mode I recommend is Arena. And targeting enemies on half health.
In my opinion the best guardian to use to get this achievement is Ymir as his passive allows his basic attacks to deal 100% damage to a target afflicted with frostbite.
So frostbite is afflicted onto an enemy when they are hit by any of Ymir's abilities (except his Ice Wall) which is good for blocking someone who is on low health.
I will write a recommended build then explain some of the items and how they work.
The build I recommend in this order is Shoes of the Magi, Polynomicon, Rod Of Tahuti, Void Stone, Rangda's Mask, Soul Reaver & Blink Rune as your Relic which is a short ranged teleport but can only be activated if you have not been in combat for the last 2 seconds.
Shoes of the Magi give Movement speed, Magical penetraion + Power
Polynomicon has a passive effect after you use an ability your next basic attack within 8 seconds will be boosted 75% of your total magical damage. which is great for Ymir as hitting someone with an ability make your basic attacks hit them for 100% damage. (so that's a great combo)
Rod Of Tahuti PASSIVE - Basic Attacks and Abilities gain 25% additional Magical Power against targets below 50% Health. This item coupled with Polynomicon & Ymir's passive frostbite literally one shot people late game
Void Stone makes enemies who are close to you lose 55 magical protection making their defence a little weaker.
And the rest are penetration and magical damage boosting items.
Good Luck.