What? You actually shouldn't even create projectiles like this because it will create it multiple times (for each client).. try it this way:
function createMissiles()
missileMarker = createMarker(355.373046875,1944.513671875,17.640625,"corona",1.5,255,0,0,255) -- create red corona
end
addCommandHandler("missile",createMissiles)
function triggerMissiles(hitPlayer,matchingDimension)
if source == missileMarker and hitPlayer == localPlayer then
local x,y,z = getElementPosition(hitPlayer)
setTimer(function()
createProjectile(localPlayer,20,354.0361328125,2027.9912109375,26.000198364258,nil,localPlayer,0,90,0,1,1,1)
end,250,0)
end
end
addEventHandler("onClientMarkerHit",getRootElement(),triggerMissiles)