That itself makes no sense, as there can be multiple units of a unit type. If you mean a unit group containing all units of a certain type that is pretty easy. If you mean a random unit of a unit type that is also easy, just a step up from the unit group using a pick random unit function. If you mean to run actions on all units of a unit type then that is also easy, get the unit group and put it in a unit group loop and run the action in the loop with the loop unit.
If this "Annihilator" is owned by AI then a tactical AI function could be written to fire its Shadow Cannon, much like how Ghosts fire Snipe or Vipers use their pull when owned by the AI. As long as the unit is part of an attack wave it will still progress towards the attack wave target while using all of its abilities as decided by the tactical AI functions.
Here's another part that I had to work around using the "grab unit" function. A pylon is being guarded by four zealots and each time one dies it is replaced. At the end of the level a warp prism loads up the remaining four zealots. So I used the "Change owner" action with the "grab unit" function and "add unit to unit group" action to individually add each zealot to a unit group to be picked up by the warp prism. I am guessing there is a neater way of doing this.
Deregister the bullies from the AI and then pick them with an area search from around the pylon and order them into the prism.
That itself makes no sense, as there can be multiple units of a unit type. If you mean a unit group containing all units of a certain type that is pretty easy. If you mean a random unit of a unit type that is also easy, just a step up from the unit group using a pick random unit function. If you mean to run actions on all units of a unit type then that is also easy, get the unit group and put it in a unit group loop and run the action in the loop with the loop unit.
If this "Annihilator" is owned by AI then a tactical AI function could be written to fire its Shadow Cannon, much like how Ghosts fire Snipe or Vipers use their pull when owned by the AI. As long as the unit is part of an attack wave it will still progress towards the attack wave target while using all of its abilities as decided by the tactical AI functions.
Deregister the bullies from the AI and then pick them with an area search from around the pylon and order them into the prism.
The event you are describing does not make any sense...
Please explain what you want the trigger should do. People could then suggest possible events, conditions and actions to implement it.