If there are no cards left in your Deck, you cannot activate “Damage Condenser”.

You cannot use “Damage Condenser” to Special Summon a monster whose ATK is variable or undetermined (“King of the Skull Servants”, “Maju Garzett”, etc.)

Discarding a card is a cost to activate “Damage Condenser”.

“Damage Condenser” is activated during the Damage Step immediately after you take battle damage. Because “Kuriboh” is activated before this (during damage calculation), you cannot discard “Kuriboh” to prevent damage and still activate “Damage Condenser” when taking damage for the same attack.

If your opponent attacks you directly with “The Bistro Butcher” while you have no cards in your hand, you cannot draw 2 cards for “The Bistro Butcher” then discard to activate “Damage Condenser”; you've missed the chance to activate it.