Coldheart is a rare Dark Elf champion and the best rare champion in the game. Her A3 ability, Heartseeker, decreases the enemy's turn meter by 100% and is one of the hardest hitting abilities as its damage is based on the enemy’s MAX HP.
Additionally, she has a 4 hit A1 skill that reduces healing by 100% when she is ascended, which is great for the Spirit Potion Keep boss and the Fire Knight. Her turn meter reduction debuff is great for Fire Knight too, while it will be very helpful for the Spider Den and the Fire Dragon. Her 4-hit A1 is also a great asset for the Fire Knight to get rid of his shield.
Make sure your Coldheart has enough accuracy to land her turn meter reduction ability (multiply the level of the Dungeon you are facing by 10). She has a base CRIT RATE of 15% at 6 stars so she doesn’t need a lot of CRIT RATE to guarantee a critical hit.
Make sure to build her with CRIT DMG instead, and enough HP% so that she won’t be the first champion targeted by enemies.
It is worth spending as many rare tomes as you need to at least max her A3 ability.