How hard is for you to hit the target is how well you see it which depends on the angle of visibility. See the pic below, the left 'person' sees the other one just as well as if there was no stone and can attack normally, while the one on the right has a limited view and has to contest a higher AC. In this case it is a side view (shooting above the stone) but it works the same way if it were a top view (say, shooting from behind a tree).
That's essentially why arrowslits work.
Almost any creature could escape before suffocating
In most circumstances, the victim would have ample time to chisel their way out.
Wall of Stone creates 10 foot panels (or larger). So let’s assume: (1) your spellcaster trapped their victim in the minimum size, 10' cube, and (2) they concentrated on the spell for 10 minutes, making the stone permanent.
I’m not aware of any rules that would cover suffocation in these conditions, so let’s fall-back on real-world facts. This Friday Fiction Facts: Trapped in an airtight room! article calculates it would take hours]for a person to suffocate in those conditions
“A moderately active or stressed person” would have about 17 hours until they experience symptoms such as “panting, dizziness, severe headache, vision disturbances” at which point we will assume they can not longer effectively chisel at the wall.
The wall is an object made of stone that can be damaged and thus
breached. Each panel has AC 15 and 30 hit points per inch of
thickness.
At 30 HP/inch times 6 inches, the victim needs to do 180 HP of damage to the AC 15 wall to escape.
Let’s assume our victim is a human commoner with no strength bonus. We’ll even take away their club listed in their stat block, and just give them an improvised weapons, with which they get no attack bonus. Each round, they can do 1d4 points of damage to the wall if they “hit” with a roll of 15 or higher.
With those assumptions, the commoner’s average damage-per-round would be 0.75 HP. On average (and with this many “rolls,” most attempts would be very close to average) it would take 240 rounds to chisel out.
A round is 6 seconds; there are 600 rounds in an hour, so our mild-mannered commoner can chisel free in 0.4 hours, or 24 minutes.
- (240 rounds) / (600 rounds per hour) = 0.4 hours = 24 minutes for a commoner to escape
So even if we make assumptions that our victim needs to rest three-quarters of the time they still get out with hours to spare.
If you are trapping an armed and dangerous creature, they are likely to be able to escape in even less time.
Edge cases
It’s easy to come up with edge cases where someone would not be able to dig out, or if the wall were made double thick, or more (which would require a lot of time since you have to concentrate for 10 minutes for each effect). In these cases the rules pretty much silent (the Portable Hole mentions death by suffocation in an enclosed space, but that’s a pretty different case).
Would there be tiny cracks in the wall, or the floor beneath it that allow air to seep in (like there are in the building where you sleep)? I think that is simply up to the DM. This is a world where you can breathe miles deep in the Underdark — the whole “how do we breathe” issue gets a little glossed over.
Best Answer
Two things in the rules describe how water interacts with stuff
Below are just some places throughout the rules that may provide you with guidance for how to deal with your given situation:
The "Underwater Combat" section includes the following:
The wall of water spell includes the following:
Besides these, how water may or may not provide things such as obscurement or cover it going to be up to the GM. When the books describe cover they refer to solid objects and not things like water so using those rules for water is certainly in the realm of homebrew/houserule.