Attack & Build, at the same time with the same unit, is a simple but very useful trick. Any builder with weapons can do it, except Thirsha and Dark Priest. Let's take the monarch Elsin as reference.
First, start building anything, for example a gate. While Elsin is building it, click again on the gate you are building and Elsin will continue building it and will start attacking if:
- he's on defensive (sometimes offensive works too);
- there's an enemy in the range of his selected weapon.
You can also Ressurect & Attack, Clear & Attack, Repair & Attack.
A ghost is a building waiting for the start of its construction. It dies with any attack, costs no mana, leaves your builder free to receive new orders, and lasts 10s. Normaly it wouldn't stay 10s because your unit would start building in less than a second, as soon as it turned and faced in the direction of the construction.
But it's possible to make it stay up to 10s. And learning how to do so makes you more skilled in blocking the movement or attack of enemy units.
Select a builder, order it to build something new, adjacent to your builder, and interrupt the process before your unit reaches the position to start the construction. Done, there you have it your cheap and mysterious Ghost (or shadow, as some players call it).
And how do you interrupt the beginning of the construction? Just move away (clicking somewhere near with your mouse) or, better, press S (stop) in your keyboard. Pressing S is a better strategy because a experienced ghost maker will be able to make many ghosts faster. Although it all depends on what works better for you and your machine.
This trick can be performed by any unit able to build. Although it's harder to be done by flying units because you it doesn't have the time a ground unit would have to turn and reach the construction (especially if your ground unit builds behind itself, needing more time to turn). A flying builder (Thirsha, Dark Priest or the aramonian Pegasus) starts building almost instantly so you have a lot less time to interrupt it's initiation.
But it's almost instantly, so you have those few milliseconds to press S, which means you'll have to press S almost at the same time you build (actually a few milliseconds after you build). It's very difficult even for very experient users, and takes a lot of practice and "feel". If you master it, it means you are already a very good player and, with a lot of concentration and sweat, you may even win Mon Fights with Thirsha.