I can't remember if I posted this in here, but I'll spam it some more

I have challenge FAQs up till Aran Ryan posted on the game page. I'll be adding more as I finish up the challenges.
http://www.vgchartz.com/games/gamefaqs.php?id=27314

Some stuff on Don

First famousringo is right. His toupee will be knocked off at around 1/4-1/3 of his health after the second knock out. However, if you beat him quickly enough, it won't come off, and you'll TKO him. That is if you are doing a lot of star punches.

Getting stars is easy on him. Just punch him before he does an attack. There are two that are hard. The first is when he does his three hooks, dodge left. If you dodge them all, he'll tire out, and then you can him him in the face for a star. The other is when his toupee is knocked off. When he does his delayed hook, counter it. To get this one, make sure to knock off the toupee early in the second round.

Here's a video that blew my mind. Via gonintendo, 4 infinite punch loops:

I just got all 3 challenges at the same time for Title Defense Von Kaiser on the first try and within 1 round

So, I beat the secret character and I'm 5 people into Mac's Last Stand. Oh, and difficulty list

1. Soda Popinski-He's by far the hardest and is almost impossible if you don't TKO him in round 1. He gets too fast and his bottle heals all of his health. He is also one of the fastest fighters already (Sandman is a little faster).
2. Mr. Sandman-He would be #1 if he didn't have an easy pattern, easy to dodge Dream Land Express, and the fact he's pathetic after two knockouts. Of course, you have to know what your doing to take advantage of this. Lot of fakeouts
3. Bald Bull-Tricky attacks and he deals a lot of damage. Fairly easy to get stars on and isn't too bad. More fakeouts then quick attacks (which Mr. Sand has both of). You also need stars to KO him, which allows for more mistakes on your part
4. Super Macho Man-Not as easy as contender, but not as bad as the rest of World S. He adds a lot of new attacks that are dodge dependent and quicker attacks. Easy to TKO in round 1, and it helps as he changes his pattern a lot. (Arguable with Aran Ryan)
5. Aran Ryan-Trickier fighting require more dodging and a whole new set of attacks. He also has a weapon attack now and can attack you after losing all of his health. (Arguable with Super Macho Man)
6. Great Tiger-Despite he tells you which way he goes, he can be tricky and has a lot of new attacks. Hard to get stars off of, and his new attack can be tricky to dodge and can do a lot of damage.
7. Piston Hondo-His Hondo Rush is more deadly and may onyl give stars when hit. He can be comboed very much after attacks. He also tricks with his fake taunt. Good at faking out.
8. Don Flenminco-Easy compaired to a lot of others. Exactly the same, save he moves into other attacks after his first attack. Overall, the same. (Arguable with Bear Hugger)
9. Bear Hugger-Realies on fakeouts. Gives stars pretty easily. Can be tough if you don't land star punches.
10. Glass Joe-Easier one you learn him, but he is good at faking out and his jabs can be fooling. He also can't be him in the head. Attacks do decent damage. (Arguable with King Hippo)
11. King Hippo-About the same once his manhole is knocked off. Opens his mouth less. New attack is easy to dodge. Still goes down in one knockout. (Arguable with Glass Joe)
12. Disco Kid-Harder then 13 namely becuase his attacks are harder to dodge, namely his three punches.
13. Von Kiaser-Easy save for his one-hit knocke out. But, it is easy to dodge and allows you beat him up. Nothing special outside of that.

I haven't faced Mr. Sandman yet, but Bald Bull has seemed to be the toughest for me so far.  It took me forever to time his Bull Rush attacks!  Aran Ryan was also very difficult for me.  Soda Popinski seemed easier for me because I beat him in the first round rather quickly.  Super Macho Man was fairly similar comparitively speaking.

Glass Joe is a piece of cake if you realize you can infinite punch him with star/5xjab/hook combo. And they make it semi obvious too by making the helmet flash yellow.

PorkChopTD: Bald Bull wasn't so bad for me. He didn't have too many tricky attacks save for his whirling punch

@nordlead: Glass Joe not higher because he is easy once you learn him, but he has a higher learning curve than most characters. Stars are hard to get off of him as well, as he rarely does his taunt.

Well, I take comfort in knowing that I beat what some consider the toughest fight in the game. Soda Pop gets pretty tough when he ignites that plutonium drink the mad scientists give him, but he's usually weak enough that I can survive if every third punch of his lands.

I would move Bear Hugger so far up that list. Maybe I was stuck in a rut, but it took me fewer tries to beat Aran Ryan and anybody else below him than it took me to beat Bear Hugger.

I'm at Bald Bull right now and he's really giving me a hard time with his different timing on the rolling puches. Soda Popinski wasn't really all that hard to be honest. He's pretty much like in contender mode. Once you hit him with a star punch he goes berserk and gets ultra predictable.