If you want to see what a boxer with "Kung Fu" would look like, that is how you do it. You can get skilled enough to be able to overcome a 100 lb weight difference, the heavyweight fighter being basically maxed when it comes to taking and giving punishment. If skilled enough, size truly will not matter.
To decline a challenge without losing popularity, first turn autosave off. Then, exit Legacy mode without saving. When you load your legacy again, your popularity will be restored and you have no record of and will not be challenged again.
To max out your stats, in Legacy mode, win the first tournament. Go to punch totals for the previous fight and press B. You will receive +5 stats for Power, Stamina and Heart. Repeat until the stats are full.
Set the round time to 1 minute and up to only round 1. Simply punch him enough to win by decision. This may also work for the "My Style Is Impetuous" trophy. Do the same while fighting Mike Tyson, but with a created boxer.
Set the rounds to 2 minutes and select someone like Foreman. Continue blocking and punching. If done correctly, you should knock them down and get the achievement after the fight.
It is recommended that you play the training games outside Legacy mode, preferably using a created fighter. That way, you always struggle. When playing the training games outside of Legacy mode, here is the rhythm to follow: go through heavy bag combo, heavy bag push, double end bag and maize bag, in succession. Then, repeat the process four more times. When you have gone through all the bags, play both sparring games three times each, that rhythm and pattern mimics a "boxing training week". What you just did counts as "one week", if you want you can keep a "training log" and spend "10 years" training (game time).
Playing the training games on your own after completing a career is always a good idea. The only real weakness fight night has is the inability to play from right to left. Although you can select that side, the problem is you cannot train from that side. It makes it so whatever player is on the left side of the screen has the advantage.