I think it may have something to do with the name of the npc being incorrect
Code:
protected void increaseKillStatistics(Player killer, String name) {
System.out.println("name: " + name);
if (killer.increaseKillStatistics(name, false) != -1) {
killer.sendMessage("You've killed a total of " + Colors.red + killer.increaseKillStatistics(name, true) + ""
+ "</col> x " + Colors.red + name + "'s</col>.", true);
}
int amount = killer.increaseKillStatistics(name, false);
if (amount % 100 == 0 && amount != 0) {
World.sendWorldMessage(Colors.gold + "<img=6> [Kill Count] </col>" + Colors.red
+ Utils.formatPlayerNameForDisplay(killer.getDisplayName()) + "</col> has reached " + amount + " "
+ Colors.red + Utils.formatPlayerNameForDisplay(name) + " </col>kills milestone! " + "<img=6>",
false);
}
Try add the print line to the method, see what it prints out when you kill the slime.
Also can you share the "getKillStatistics" method in the player class