When played, Undyne's Spears will deal 5 DMG split into five instances of 1 DMG hits among enemy monsters. The targets of each hit are random--as such, it is possible for one monster to receive the full 5 DMG, or for a monster to avoid taking any.
If a monster reaches 0HP during the effect, it will not be eligible for any further hits. If all monsters on the opponent's field have 0HP before all 5 hits are dealt, the effect ends prematurely, with the unused damage being wasted. Dust effects do not activate during Undyne's Spears, however other effects that rely on a monster taking damage will go off for each hit.
Undyne's Spears are excellent for cleaning up the opponent's monsters of their health is low, and can be used in conjunction with other Justice damage spells and the Justice soul power to ensure a board clear.
Conversely, they are incredibly poor if your opponent has a single monster with a large amount of HP, as the spears may hit that monster and reduce the destructiveness of the card. If faced with high HP monsters, directly modify their HP to a lower value with cards like Chaos Duck so that they are easier to deal with.
Dust effects do not activate during the damage dealing, so do not count on destroying a monster summoned by a Dust effect when using Undyne's Spears.
Using Undyne's Spears with Same Fate can quickly clear the opponent's board, but possibly at the price of your own monsters as well.
As each hit counts for only 1 DMG, cards that ignore small amounts of damage such as Mad Dummy, Armor monsters, or Diamond Boy 1 will render this card completely useless, as they are able to soak up the damage at no cost to their own HP.
In a pinch, Undyne's Spears can be paired with Asgore to damage most, if not all, of your opponent's monsters for Asgore's effect.
- Spears of Justice (by Diamaincrah)