Think of Brickwalls as a test. A preliminary round before the actual fighting. You force them past your wall before they get to actually do battle with you.
This is it exactly. Or more wordily, a brick wall is a "you must be this tall to ride this attraction" sign. I see you have a little Yoshi badge (Opfer), so take Yoshi's ledge-camped up-B. You get some invincibility frames for grabbing the ledge and you just pelt them with projectiles that do pretty decent damage. If you enter a match against someone who has no clue how to deal with this tactic, you could repeatedly use it -- possibly even to the point of winning by relying almost exclusively on ledge-camped eggs. The tactic of "edge-camped eggs forever" is a brick wall: if your opponent cannot get surpass it, they cannot "actually" fight you. They weren't good enough to ride the "have a normal battle" attraction.
Some would call this "spamming", or "being cheap", and by their definitions, they are probably correct. And you may choose to avoid spamming or being cheap: maybe you are playing for fun, or playing to learn, or playing to please the crowd, or playing for some other reason. But if you are playing to
win, if winning is your primary and only goal, and you can win via a brick wall, then **** son, you construct that brick wall and you win with it.
But maybe your opponent is good enough to answer Yoshi's ledge-camped eggs. At this point you can pull out another brick wall (if you have one), like the pivot-grab vs any ground approach and the u-smash vs any air approach. And if your opponent cannot beat this new brick wall, then (again) you've basically won the match.
If your opponent can answer or beat all your brick walls, then you have to "actually" fight them. You can certainly still use pivot-grabs, u-smashes, and eggs from the ledge, and you likely will do all these things even multiple times in a row, but you (probably) will not try to restrict the match to one of those brick walls again. Thus the answer to your question "So how do brickwalls work against players that already know how to deal with them?" is "they don't". If your opponent can prevent you from turning the Brawl match into a "try to get around my ledge-camped eggs!" minigame, then you must fight them on more even footing.
Hope this helps,
/RtEB
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