I’ve made a rule that runs when event:player dealt final blow happens.
it has total of about 2 seconds runtime.
And I’ve noticed if a player gets kills simultaneously without delay, the actions would only run once because the actions would not overlap-run.
Any advice would be appreciated.
I don’t think this has happened to me. Perhaps set certain rules (12 for ffa, 6 for teams) and have it have the condition
Slot of Victim = Slot (The rule number, like the 2nd rule is 2), or something like that.
As I said, this hasn’t happened to me, so this might be the case of your wait command being Restart If True.
Use Player Died and change Event Player to Attacker, this will solve your problem 
Rules will pretty much only execute on one thread per player and you can’t have the same rule running multiple times at once for one player. Wait Behaviors can only stop the rule from executing again (Ignore Condition), restart the rule (Restart When True), or abort the rule’s execution (Abort If False).
This is a good solution because it executes once on every victim, instead of multiple times on the attacker (which causes problems). Shouldn’t cause a problem, assuming that the same victim isn’t killed twice within your 2 second rule runtime.