well i have aim too, it's up to you guys. i really don't care. it's just nice to have a collective group of us all.
i guess right when you see bowser/whatever character you're performing it on go into the blast zone [bubble or w/e] OR as he's approaching that point, you can start mashing x then.
as an example you can watch kingkong vs kingtoon game 1, although it's kinda hard to keep up with and going back and forth in youtube videos is a pain. the most advice i can give you is practice it...i can make a video on it when i get home but you'd have to wait like a week to 10 days.
well the reasoning behind the whole jumping thing is because when bowser does the bowsercide, most of the time the victim dies before him, so there's a small time frame where bowser's just chilling in the blast zone. bowser's in the air so when you "jump" out of the bowsercide, it's a double jump and you only have your up-b left.
the timing differs because the blast zones of each stage differ in length, depth etc (example, smashville as opposed to ps1 or something like that), and the size of each character differs as well, which means the time/speed they enter the blastzone before bowser and the time that they die differs. combine both together and you pretty much have a different timing for each stage but a kinda similar timing for each character size. i think, or maybe it's the other way around. for example kirby, mk and jigglypuff have a similar timing, mario and diddy have similar timings and probably luigi as well if we're going by that logic.
i can't really do it that often on larger characters though
and tell me if this makes sense at all because i've developed a habit of explaining things half-assedly at times, so sorry if it doesn't.