If a creature's power is somehow less than 0, it subtracts from the total power of the other creatures its controller chooses. This can cause creatures with power 5 or greater to survive.

As Slaughter the Strong resolves, first the player whose turn it is chooses which creatures will be spared, then each other player in turn order does the same knowing the choices made before them. Then all the creatures not chosen are sacrificed simultaneously.

Slaughter the Strong has each player choose any number of creatures and then checks that the total power of creatures each player chose this way is 4 or less. For example, you could save two 2/2 creatures, or a 1/1 and a 3/3 creature, but not all four of those creatures.