Oh yeah, I did beat Soul Melter EX finally. I did Animal Friends solo to do it. I never found the AI partners to be consistently helpful enough to warrant the increased health bosses get when fighting them with multiple characters. Rick, Coo, and Kine have all of the abilities you'll need to defeat the bosses anyway.
Rick's rolling stone form and Coo's Parasol tornado are the big winners here. A ton of invincibility while dishing out solid damage. Their guard is also enhanced, providing another excellent defensive option. With easy access to fire, wind, and even water they'll typically have a solid weakness option for most bosses. Most importantly, they can blow out Whispy's golden apples, burn Francisca's Ice Candy, and wash Dedede's food for health drops in those boss fights, as well as blow out Flamberge's fuse for when she uses her giant fire cannon.
An important ability to consider is Coo's feathers. Individually they don't do a whole lot of damage, but you can fire them off surprisingly quickly, they travel across the whole stage, and if you're up against a larger boss hitting them with two or even all three at once can quickly stack the damage in your favor handily.
On top of their defense is their mobility. Coo's flight is quite fast, and wall jumping with Rick is the fastest vertical movement in the game when available. Of course, Kine is the fastest swimmer too when water is relevant, but that'll probably only happen once. Between being immune to damage through a variety of methods, avoiding damage through excellent mobility, and healing damage at certain critical fights, this trio has an excellent survivability without the need for other allies and without sacrificing much damage other characters like Marx, Magolor, or Dark Meta Knight may dish out. Perhaps the biggest difference between Coo's tornado and Rick's stone when compared to other characters' moves is how safe they are. Rick's stone lets you exit it at any time and move around to a safe position for you to do so. Coo's tornado moves around the air incredibly freely and quickly, letting you deal damage and return to a safe position before it ends. No other character boasts nearly as much control in their invulnerability states.
I haven't yet tried RK&C, but I've solo'd Soul Melter EX with the Three Mage-Sisters, and their strengths seem comparable, as far as iframes and multiple elements go. The sisters are little trickier to handle due to the complexity of their moveset, but their main advantages over the animal friends are the fact that they have access to electric attacks, excellent dodge rolls, and a screen-nuke attack which charges back up roughly every other boss fight. RK&C is probably a safer defensive pick whereas TMS is better at dealing damage and disabling bosses.