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>]

you could do /tp @e @p to tp all mobs to you not sure if that is what you are looking for

The exact command is this command below. The commands before "summon" is for 1.16.x and dont know if it works on 1.11 or not If you know how 1.11 works you can use this pattern on 1.11 or what version you want:

/execute at @billie run 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}]}]}