I remind you that when concentrating on the external world, you should also be fully aware of your body (see Zen Training : Methods and Philosophy by Katsuki Sekida.) Otherwise, no special skills are required ift when focusing on something, you lose the feeling of your body.

And you can really be aware of your body only by denying the outer world - the context - zazen in fact is a total denial of everything. That's why spiritual practices have always attached great importance to such challenges as dousing with cold water, for example. Otherwise, you can easily concentrate on a sexual object, but it will dissolve your self - to what 'elimination of dualism' could this possibly refer?