I know what he means. For starters, this Wesley guy is the one posting all those Powerpuff Girls posts (and admits he has autism). For the Synergy assist thing, he's talking about something that can be done in the Super Smash Bros. rip-off Cartoon Network: Punch Time Explosion XL.
In said game, when a character releases an Assist Cube (exact same thing as Brawl's Assist Trophy), there's a chance that instead of the assist doing it's action, it'll team up with the person who released him/her and basically do a second Final Smash. For example, if Chowder releases a Madame Foster assist, his Synergy attack might activate, where a cutscene shows Madame Foster giving Chowder a huge pile of cookies, Chowder eats until he's a fat ball, and the Madame Foster jumps on Chowder, then rolls as the player crushes his opponent. A really cool idea if you ask me.