You need to use the calculation of distance and damage.
The farther the player, the least damage.
If the player is close, give him high damage.
There are many plugins who use that calculation, search.
P.S: Something like this:
PHP Code:
// From Molotov Cocktail plugin.
tempdmg = damage - (damage / range) * get_distance_f(origin, pOrigin);
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