Endgame Coldheart is supposed to be all about damage (crit rate, crit damage, speed, attack) and accuracy, but mine still uses HP% chest and Def ring and banner. Her main damage to dragon is A3, that doesn't scale well off Atk, so crit chance and crit damage are more important.  IIRC, when I was at dragon-13, my Coldheart even used Def% gloves. She was focused and couldn't get to Dragon without defense/hp gear and Vrask in the team. More HP may mean that waves will focus others. Also, replace Relickeeper with support/healer that will keep your Coldheart alive.

Summarizing Coldheart stat priorities. Get enough HP and Def to survive. Then get Speed, Crit chance and Crit Damage. Then 100-200 ACC (not needed at all for dragon). Then Atk. I believe Atk is one of the least important stats for early-midgame Coldheart unless you really need her to clear waves with A2 and A1. Crit rate gloves, HP% chest, Speed boots, Defense ring.

What you need also depends on what you struggle with. Is it waves or dragon? For waves you may want more AoE, crowd control, may be someone in shield set. It it's dragon, does it kill you fast (you need more sustain then) or one-shots after several "inhales" (you need damage then).

You mostly get artifacts by farming Dragon and other dungeons, little choice there. You farm whatever dragon you can, get better 5* artifacts, then suddenly your team can beat the next one. You also work on your champions: level to 60, use books, get masteries. Then open shards and level new, better, champions.

My coldheart is far from perfect, wears two 6* items, but they are helmet and shield. Here she is. You may see that her additional Def is higher than Atk.

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I want to add a few things to what Sunny said. Firstly, Coldheart is the most important in Spider and Fire Knight, where she's doing not only her big damage numbers, but also the turn meter depletion (which the dragon is immune to, sadly) - and in the case of FK, having a 4-hit A1 with heal reduction is also great. She can work on the high stages of dragon, but she adds less value there, just the big damage numbers of Heartseeker. On early/middle stages of dragon, like stage 12, she is rather meh and I wouldn't recommend using her unless you really don't have much else, because damage scaling off enemy max HP only starts to outperform regular damage when the bosses get very high HP. For spider and FK, the turn meter depletion and heal reduction require accuracy - for stage 20, a bit above 200. 

Secondly, keep in mind that Heartseeker has a built-in extra 30 percent crit rate. Meaning that for her serious damage, Coldheart doesn't need more than 70 percent crit rate. If you push her crit rate higher than that, it's only for the A1 and A2 which don't do very much damage anyway - you should have other champions putting the big AOE damage on dungeon waves. 

So looking at Sunny's CH, I'd recommend to reduce crit rate to 70 percent, increase accuracy above 200 and increase crit damage as high as possible. 

I completely forgot about 30% crit rate increase. I will replace crit rate gloves with crit damage ones.

I've finally come to Dragon-20 and Coldheart doesn't make the cut. I put my full clan boss team there for 100% win. It's slow, it takes 5-6 minutes. Waves take most of the time, then Frozen Banshee makes short work of the Dragon. Will check FK and Spider teams when corresponding events come.

Coldheart, best use in Spider; not recommend use her in other dungeons because she is too squishy, unless you have special champs that can bring the whole team to fight boss. 

I can use her in all dungeons because I can bring her to boss. 

To build her you don't need speed, around 150 is fine.

Crit rate must be 100% in total stats.

Acc is 200+. 

High Atk, Crit dam. 

Sacrifice other stats.

Don't expect too much from Coldheart, all you need from her is her A3. Only when you are strong enough, you could use her properly. 

Coldheart only shines at higher level dungeons. Dragon 12 does not have high hp for Coldheart's 3rd skill to hit hard.

Some of the best ways to build her is with broken sets, meaning to use the filter for crit rate, crit damage, and speed. For end game stats:

200+ speed

70% crit rate

250+ crit damage

all can be aquired with broken sets, but as end game if you cant reach that high, you might need a decrease defense and weaken to the boss and an ally attack to your coldheart for her to be successful. 

Hope this helps! 

This is my Coldheart:

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My Coldheart has no Set Bonus, but don't let it fool you.

This is my Coldheart Stats.

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Yeah, my Coldheart can hit the Spider Queen extremely hard.

thanks for all the answers... it helped a lot

dang your cold hearts crit damage is nuts you would be hitting over 2 mill per heartseeker easily hey J

i need better gear best i can get is 150 odd crit damage at moment and 1.3 mil per heartseeker but now ive wiped spiders 20 after wasting so many gems and power trying different team combos ill be grinding for better gear to get my ch hitting harder 

My Coldheart has C.Damage Gloves - HP % Chest - SPD Shoes.

You want to make sure your Coldheart has 70% or More C.Rate.

Your Coldheart will gain the extra 30% C.Rate from her A3 move.

You want all the gears to have rolls in C.Rate or C.Damage

I used the Filter system.

I looked at all the Gears on my account.

I searched for gear pieces which had the best upgrade rolls in C.Rate or C.Damage.

Here is a Screenshot of another Coldheart I own.

I use 3 Coldhearts Max leveled in my Spider team so that is why I got a few different builds.

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Now this Coldheart - You can see has Speed Set Bonuses.

Obviously, I wasn't trying to get any Set Bonuses.

I was just looking for the gear which gave me the best blend of C.Rate + C.Damage.

It just so happens the gear I was looking at was in the same set.

It worked out in its own way.

This is my Coldheart Stats. 

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This Coldheart is a little weaker vs. my other Coldheart, but she does OK.

I still haven't managed to get a Dark Elf ACC Banner for this one.

The Spider likes to drop every kind of Banner except Dark Elf ones.

Here's mine, not as uber as the one above but does the job. She was geared during the time I was still farming dungeon level 16/17's. 

She survives through dragon 20 without too much trouble. She also on my Spider 20, Fire Knight 20 teams. 

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My 2 cents  specifically for spider dungeon builds , but can work elsewhere with some tweaks : 

Another way to improve CH spider builds and min/max them for damage is to rely on crit rate buffs and/or crit rate aura champions, by building a coldheart 40% crit rate and adding a 30% crit rate buff on top of it you'll be able to stack more crit damage. 

Furthermore Masteries are very important : Helmsmasher if you can get your Crit damage above 200-220% will give you a chance to ignore defense and therefore more damage.  Flawless execution if you're struggling to build up crit damage to 200%

Finally one cruel set on a coldheart can go a long way that extra ignore defense does boost the damage significantly. 

Curious whom you use for the Crit % aura. Queen Eva? 

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I two need help with mine.  I got lucky and have 5, 4 are maxed.  On spider 16 with a dec. Def they're only hitting for around 500k at .  I see videos and forum posts of 1mil+ ,  is this with an inc. Atk duff and dec. Def?

Dec def is mandatory, weaken is helpful, inc atk is useless. 

Damage to spider-20 is higher than to spider-16 because it's based on boss max hp. This is why you deal 500k. It's more than 1/4th of spider HP. You will deal more than a million to spider-20 with the same stats.

Hey, I saw people talking on this thread again.

You peeps have some awesome Coldheart builds.

But I did notice 1 thing.

Most of the Coldheart builds people are showing are for Spider & Fire Knight.

I haven't seen anyone post any screenshots of a Doom Tower Coldheart build.

Since, I showed my other Coldheart builds.

I figured I would come back to show you peeps my Doom Tower build.

I run the below build against the Scarab King.

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Here is the Total Stats:

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The Banner I am using for this Coldheart is Resistance.

The Tier 6 Mastery I am using is the Unshakeable (+50 Resistance).

This Coldheart only mission is to do Big Nuke + Resist all the stuff Scarab King does.

I don't care if my Coldheart reduces turn meter.

I don't care if my Coldheart does heal reduction or poison.