I'd like to suggest a variant with a bit more enhanced realism, in that he doesn't have to take any chances or make guesses (pro assassins would hate an uncertainty!), and also keeps very closely to the original idea, but addresses a few points mentioned in other answers/comments, if it doesn't spoil the intended plot. Also as much drama, and perhaps able to keep a reader guessing even more
Most toilets/bathrooms have a sink to wash ones hands. The sink has some interesting features for an assassin behind a wall - it's more often towards a far end if there's space, and it almost inevitably has a mirror (either over the sink, or if not, then a full size wall mirror as in some disability-friendly toilet/bathrooms). The mirror is what we want. It's separate from but flat against the wall (or flat against the sink's back panel/wooden units), and it's quite large.
Imagine the current plan. Marking an X on a wall and hoping the target isnt leaning forward, reaching sideways, holding their head bowed, reaching for thre paper or their trousers .... and if he misses and the target drops down low, how to figure his aim for a second finishing shot? No, the guesswork is far too much for a skilled assassin. But he wants to shoot through the wall anyway.
Can you see where I'm going?
Instead of a hole by the toilet or a blind shot through the wall, the assassin first locates the mirror in the bathroom. This should be fairly straightforward for the kind of assassin in the question. He could:
- use a cheap metal/pipe detector as sold by any DIY chain, to locate and trace the sink pipework
- use a long range view with binoculars, somewhere he couldn't shoot from but could view from
- somehow gets a description from a past visitor/tradesman/associate
- figures from the floor plans that most apartments have the same layout which pretty much forces the toilet in one corner and sink on the opposite wall
- drills a tiny pilot hole somewhere else, like at floor level under the toilet/sink, and passes a fiberoptic size tiny fisheye lens through it to view the layout. (My favourite of these!)
He/she then carefully cuts/drills away a fair size hole behind the mirror of the sink, stopping when close to breaking through the wall to go really slowly. This isn't hard because walls in any specific building are almost always similar thicknesses. Noise isn't a problem - from time to time anyone's neighbour will do building work and everyone will assume that's all it is. So he doesn't need to know when the target is out or in a meeting. He might use an ordinary cheap angle grinder with a diamond blade, as used by any DIY/builder, instead of a hammer/chisel, because this cuts the wall away without putting any pressure/force on it. It would take maybe an hour or two to do it. So when nobody's around to hear, he can expose a sizeable area of the back of the mirror through the wall, like a rough circle 4 - 6 inches across, and it can't be detected.
And now, remember one key thing about mirrors. You know the one way mirror beloved of police interview rooms in thrillers? All that a one way mirror is, is a half silvered mirror with the "hidden" side in the dark. The person on the lit other side only sees their own reflection.
So your assassin turns off the lights in the next apartment, and with a fine abrasive, gently rubs the back of the mirror through the wall, stopping once he can slightly see through any part of it. If his rubbing is careful and fine (and he's practiced it of course!) then he's now converted his target's toilet/bathroom sink mirror into a makeshift one way mirror. There's no sign of it (who looks hard at their sink mirror?), and it's dark his side anyway.
But when the target comes in and sits down, he has a perfect sight for his hit, down the barrel of his gun, of the target with trousers round ankles on the toilet (or standing stationary looking at it), and all that lies between them is a thin sheet of glass. Even if the target moves, the hole is large enough to see everything and adjust his aim sharply downward. And being the other end of the bathroom, its a great vantage point.
He might figure as a fallback plan, even if the toilet location was wrong (they moved it in that bathroom), this will give him a good shot anyway, when the guy washes his hands, even though toilet is preferable because of immobility and trousers obstructing movement.
Shot through the wall, and guesswork removed. Bye-bye target!