Ruby+

'The Ending' is the true final boss. After you beat the Act 3 boss, instead of leaving you take the hidden path to Act 4 and kill the Corrupt Heart. To open that path you must be carrying all three keys, collected across Acts 1-3:

• Ruby Key - skip the rest/smith at a campfire to take it instead.
• Emerald Key - beat a burning elite (the one with the green flame).
• Sapphire Key - skip the relic in a treasure chest to grab the key underneath.

Do all that as the Ironclad and put the Heart down for this one. It hits like a truck, gains Strength every turn (Beat of Death) and starts with a big Blood Shots turn, so bring a defensive deck with real burst - Strength Ironclad or a Barricade wall both work. This plus 966201847 and 902745455 earns The End?.