We have reached what appears to be the final battle with Kaos. I feel like I don't want to proceed any further because then the game will be over. We still plan on "three starring" every level and of course there are the Adventure packs, but I know I am going to want more.
Someone earlier in this thread suggested that the heroic challenges scaled to your level. My experience shows otherwise. I played through a few of them with a level 7/8 character and he was able to one-shot even the large enemies. It was far too easy to blaze through and destroy everything. Zero challenge. This is quite the opposite of when I tried them with a level 1 character, or even level 3/4. It seems these heroic challenges are exactly what Cali calls them: training. They give you zero coins and plenty of exp. They truly are training grounds intended to give you an interesting method of power levelling your low level characters.
Now I feel like sharing thoughts about some of the characters.
Chop Chop: Initially he was very weak. A melee sword attack and a shield? Compared to any of the ranged characters, he was virtually useless. However, after a few upgrades, he is one of the most powerful characters. Sword does a lot of damage. Shield Bash and absorb ability means he is interesting to play and can plow through small enemies. The Brambles are hard to aim, though.
Terrafin: I've given this guy many chances. Brass knuckles, the mini-sharks
it doesn't make him any more fun to play. He seems powerful enough, but is just boring as hell.
Stealth Elf: I don't know the point of the scarecrows, as the enemies usually die before they matter. It's also thematically odd. The stealth ability felt right, but having a scarecrow suddenly appear is very strange.
Eruptor: Once you get into the eruption upgrades, he gets very busy graphically. The eruption becomes very large and throws fireballs around. The game screen can already be confusing during large battles, and this doesn't help.
Voodood: Between his very strong melee attack and his trip wires, he is a powerhouse.
Boomer: Another character that starts very weak, but becomes extremely strong with upgrades. Triple dynamite and troll bombs are nuts. He also has interesting game play in that you can either attack slowly from long-range or quickly from short-range.
Sonic Boom: As I mentioned in a previous post, this has to be the easiest character to play. Wide area attack with decent range that hits multiple enemies. AI-controlled chicks. Flying.
Ghost Roaster: I'm not sure about the utility of his ectoplasm mode, as his skull attack does pretty much the same thing, but deals big damage. I think you have to go for the ectoplasm upgrade path, otherwise it is a pointless ability.
Gill Grunt: Top tier like Sonic Boom. Unlimited water upgrade means you can infinitely hover around in any direction while spraying groups of enemies with water. When fighting against single or very strong enemies, his extremely powerful harpoon gets the job done quickly. The fact that he can access many of the hidden items and areas across water is a bonus.
The rest of the characters are all good in their own way, but don't really stick out in terms of power level or utility.