Hand of Greed

Hand of Greed is perhaps my favorite.  It shows 20 HP damage below, though as a note its upgraded version does 25 HP and provides 25 Gold.

Any non-minion target is viable.  The Hand of Greed is repeatable, unlike the Feed card.

Notes

• Repeatable. 

Event Synergy

• Arena event.  This event allows you to fight and potentially consume x5 non-minion creatures.

Monster Synergy

• Best used in multi-monster encounters, where you can take advantage of consuming more than one per encounter.
• Any monster group of three (3) - Several of these exist from slimes to cultists, byrds, etc. 
• Darklings.  Also in a group of three.  Note, however, it doesn't look like you can take advantage of their return back to life to consume them multiple times.
• Gremlin Gang.  Usually four of them.  This can provide 80 to 100 gold in an encounter.
• Slimes.  When slimes divide, they are noted with an (M) next to their name.  These are not valid targets for the Hand of Greed.

Feel No Pain

Whenever a card is exhausted, gain 3 block.  You can read more about the card here on Fandom.

Notes

• Not affected by dexterity or frail.

Card Synergy

• Awesome with shivs and other weapons that get exhausted.  Same for cards that exhaust but provide other (exhaustible) cards as well.
• If combined with Dual Wield, it can be used to constantly create 1 (2 with upgrade) of exhaustible weapons to create an endless supply.
• If combined with Infinite Blades card, provides a new shiv every turn.
• If combined with Second Wind, can exhaust all non-attack cards in hand and provide 5 block per card in addition to the 3 Block from Feel No Pain.

Relic Synergy

• Ninja scroll lets you start a combat with 3 shivs.  
• Dead Branch adds a random card to your when a card is exhausted.
• Charon's Ashes will do 3 HP damage to all enemies when a card is exhausted.
• Medical Kit will let you Exhaust a status card.

Monster Synergy

• When fighting the Elite Monster Sentry, the "daze" cards they put in your deck will exhaust, adding block to your total.
• When fighting the Chosen or enemies that place a "hex" on you to add "daze" into your deck, it will add block to your total.