darkSpyro

For the PS3 and Xbox versions respectively, there is a trophy and achievement called "One For All". In order to get it you must defeat Kaos without swapping out Skylanders even a single time. I got his health to about 35% (before I wiped out) with Sonic Boom at level 10, with the highest armor hat I had, and her abilities fully upgraded down the sonic blast branch.

Does anyone have any tips or tricks for this trophy? Any particular Skylander preference for it? If so, why?

I haven't done it YET with only one, (don't think PC version has that achievment). But I did manage to do pretty much the entire battle with a maxed up stealth elf. The only reason I didn't do it all with her was cause I started with Gill Grunt and Trigger happy, who both died...

I can get rid of all 3 of the minions in a few seconds with her. She's a bloody TANK.

If you have lots of the characters, pick your favorite character you want to play with, and finish as many of the heroic challenges you can. I found Kaos very easy to beat, even without swapping because all of my characters have finished all 18 of the challenges i have unlocked. The upgrades you get may seem insignificant, but they add up quickly.

Terrafin, especially when taken down the Sandhog/shark/whateveritscalled path.

He's an absolute beast, and can't be damaged by any of Kaos' spells while he's burrowing.

Easymode.

Also, if you have the Pirate Seas and/or Darklight Crypt game pieces, put them on while Kaos' minions are attacking and it will wipe them out after a few seconds. Something I didn't find out until I found something online about it. The game pieces don't just unlock the level, they are practically smart bombs.

I got the achievement last night with Spyro and just kept using his fireball attack whilest staying well away from Kaos and the minions. Here's a couple of tips:
When you defeat one of Kao's minions they drop health food. Don't eat the food if you are at full health. It doesn't disappear so eat it when your health is reduced to about half.
When you run out of food you can restore more health by running at the side of one of the sharks the water dragon summons. The screen will slow down and you will begin to heal.
If you see the fire dragon getting ready to attack get right into a corner and you'll avoid it's fireballs. Also get into either the bottom left or right hand corner when the undead dragon summons the lasers and you'll have a bigger gap to avoid the lasers.

This is all very very helpful. Thank you so much!

Apparently Trigger Happy can wipe him out at lvl. 10.