well, as a battleships map maker myself, i can tell you this. the only way to focus your fire on one unit using phoenix fire is to well, making it only target a specific target group (i.e heroes)
there is no way for a phoenix fire spell to target a specific target repeatedly over when it is within a group of units.
However i did try one way that allowed you to target the closest enemy to you repeatedly using dummys but it was buggy.
But apart from this, the only way as above is to make it target a specific unit group OR create a trigger that makes all your attacks spawn a dummy that attacks the specific target with a buff, but that is not the point of phoenix fire.