A meditation on a subject matter, say reflecting the Fruit-of-Manukau's telepathic attacks on me, requires the subject matter held and maintained in my conscious thoughts. This can be almost impossible if the attacking group outnumbers you as one of that group can try to shift your mind onto something else and the other can make you forget what you were thinking or doing in the first place.
One method to overcome this is to put the meditation and maintenance (ie focus) of that subject matter into a conscious action, say crossing ones fingers or holding the foot in a contracted position. You can now do anything else. It will seem strange at first but this method actually works.
As to the subject matter, it can be anything, one thing or a whole range of things. I have been told that this is also called prayer. So for me, I now do this constantly and choose the subject matter to be all good things.