Undyne's Spears are a Justice deck exclusive effect card, they work similarly to the Magic effect of Undyne. These will not kill Mad Dummy, due to them doing 5 × 1 damage. They can work good with Asgore.
Counters: As mentioned before, Mad Dummy will be able to absorb the attack with no loss. The same goes for the allies of Diamond Boy 1. Monsters with high HP such as Greater Dog and Royal Guard 1 might be able to recover from the damage in a Kindness deck, and Asriel can restore surviving monsters back to full health.