All you do is make a tag value on the hit player then when he dies the tag will determine the player who killed him and will transfer his time value and add it to yours.
I suggest reading up on scripting before jumping right into the deep-end. This category is designed to house people with initial scripts that need support, not for people to spoon-feed code.
You can find resources here to help progress you:
If you are looking for someone to develop a script for you, please use the category sub-category hiring.
What you are looking for in general is binding a .Died event to the players humanoid
Humanoid | Roblox Creator Documentation to fire an event when ever a user dies. You can check for a instanced value, often named Creator inside the player’s character. This should be created when a player is damaged by another player, so you can tell who attacked who, as Roblox doesn’t have any method of telling you this. This creator script could be embedded in the sword script.
You can then access the leaderstats and manipulate the values based on this event.