The Corrupt Heart is the final boss, fought in Act 4. It is very powerful; its innate powers and buffs it gains throughout the fight can prove very difficult to counter. Because of its Invincible buff, it can only take 300 (200) damage per turn, rendering strategies that rely on heavy damage less effective. Additionally, it will gain incredible amounts of Strength on its fourth buff and onward, so killing it quickly is also important.
Any source of Weak will also be extremely valuable against both of The Heart's attacks.
Sources of damage negation such as Intangible and Buffer can be very effective against the Echo attack. Intangible is also reasonably useful against Blood Shots, as it will always lower the damage to 12 (15), whereas Buffer would only negate 1 (2) of the attacks.
Thorns is also very good due to it triggering 12 (15) times for each Blood Shots that The Heart does.
Anything that triggers on-damage or on-attack is very good against The Heart due to it triggering 12 (15) times for each Blood Shots that The Heart does. This includes:
Medical Kit can be useful against The Heart, as it will allow you to remove the many statuses it will inflict on you, but beware of its Beat of Death power, as it will trigger whenever you play any card, including curses and statuses.
Tungsten Rod and After Image can completely negate Beat of Death 1. Both of these, or 2 copies of After Image, are necessary to negate Beat of Death 2, applied on The Heart's second Buff action.