We all know skeletons have superior aiming skills in Minecraft (well, not really, but Infinite Arrows is still a strong argument), so it only makes sense to have these guys guarding and shooting enemies for you in a radius of 17 blocks. This array is made by adding an Arrow (Reagent) and then a feather (Catalyst) to a blank array. This will create a trap:
Now all you need is a skeleton to walk over it, this will trap the skeleton and make the trap skeleton treat you as friendly: it will spontaneously attack any nearby hostile mob (except Skeletons)
It is a good idea to protect them with a wall or a fence and also to keep a block over their heads so they do not burn in the sunlight, a name tag might also be a good idea so they do not despawn. If you push a trapped Skeleton out of the Array, it will still act as a friendly sentry and remain perfectly static, even when someone hits it. If another Skeleton walks through the Array, it will not be affected and remain hostile because the first one is still possessed by the spell, so you can’t use one Array to trap more than one Skeleton.