Does the Brace "stance" happen immediately, or, like a Readied Action, happen when triggered, which, in this case, is the Charge?
So, you ready an action to Brace. You don't change stance, or anything, like a normal Readied Action. Enemy Charges, the Readied Action(Brace) triggers, you deal double damage.
Wild boars charge, and impale themselves. That's what they do.
Actually only one charges into the spear (that would be Wilbur) the rest of us are tearing out your knee ligaments, of course that's not where the true value of us Boars lay tho, there's a reason French people call us "Truffle Hounds" (i mean besides the french being a~$#+%$s and needing to come up with dismissive nicknames for anything not french).
So think of that next time you dine out at a fancy restaurant some stinky f~*+ing pig found that truffle you paid so very much for:-)