Summon mobs near player/ Move to player

The execute command should do the trick. Here is an example that spawns the mob you were trying to spawn at the location of every player.

/execute @a ~ ~ ~ summon spider ~ ~1 ~ {Passengers:[{id:skeleton,HandItems:[{Count:1,id:bow},{Count:1,id:shield}],ArmorItems:[{Count:1,id:diamond_boots},{Count:1,id:diamond_leggings},{Count:1,id:diamond_chestplate},{Count:1,id:diamond_helmet}],CustomName:Killer,Visible:1,HandDropChances:[1.0f,1.0f],ArmorDropChances:[1.0f,1.0f,1.0f,1.0f],ActiveEffects:[{Id:1,Amplifier:1,Duration:999999},{Id:5,Amplifier:1,Duration:999999},{Id:8,Amplifier:1,Duration:999999},{Id:12,Amplifier:0,Duration:999999}]}]}

Of course this can be changed to spawn any mob like this:

execute @a ~ ~ ~ summon <your mob> [x] [y] [z] [dataTag]

Also, if you wanted to summon it at the location of the nearest player you simply change the @a to @p

Or to do a specific player enter @a[name=<player name>]