Can you hide behind the illusion? Probably. Can you get the advantage just by being behind this? Probably not, unless you're a halfling.
Here is the problem, ultimately, you are only creating a cube of 5 & # 39 ;. That is probably not enough to hide without a real stealth check.
Basically, you need to completely block the line of sight of the creature in which you want to gain an advantage, for yourself. Also, theoretically you need keep said blocking of the line of sight to your opponent until the moment you attack …
The way this probably It works best if you are a halfling and throw this the previous turn. Then you could duck and shoot through the illusion with advantage.
For a normal-sized character, without an action to hide, he probably wouldn't allow you to remove yourself from LOS behind the illusion (that doesn't mean he doesn't block). Most of the characters are over 5 & # 39; tall (a huge Strip of humans are, as probably 80% or more, and probably even more adventurous humans). That means that even if you only occupy a 5 & # 39; cube in the game space, they will see you above an illusory one. You would cover yourself … but the thing is an illusion, so if they shoot you, it would give the creature the impression that you have coverage, but none would apply to the attack (or maybe half the coverage number, then -1 or -2 instead of -2 / -5). However, the advantage in their attacks is not so much.
However, if you are willing to spend the action (to do a dex (stealth) check to hide, you would recognize it, because you are actually spending an entire action, and that is modeled by being quiet, taking a gentle step, and possibly crouched behind an illusory object.
So, yes, the wall is opaque, and you could hide behind it / take advantage of the attacks (technically, if it were a real object, they couldn't point at you … It would allow the target) … but only if You fit completely behind him, and that's not the case for most PCs.