Sniper tower.

This would be a defence which would have a very long range like the xbow set to ground however it would target both ground and air.

The player can set the defence to target a specific type of troop for more damage and would priotise them although attackers wouldn't be able to see what it is set to. This would help defenders where their base seems to always struggle against a particular type of troop.

It could have high damage but a low fire rate, firing at the same rate as a mortar but doing 300 damage per shot to the set target at level one.

This would first become available at th8 where it would be extremely useful as mass strategies such as hogs and dragons become popular and it would hopefully encourage more creative attacks.

It would look like the sniper from dota 2, and be set on maybe a camouflaged version of an archer tower.

One of the uniqueness of the X-Bow is that it has a long attack range. Having a defense with the same radius or greater would take that away from it. Sounds OP that you can priorities its target.

This is basically the way you described it as an eagle artillery and sounds way to OP.

Too OP for defense that have x-bow range,target both air and ground,equal damage to lv.8 AD and can select target.
This should be for TH12 not TH8.