There's no-online, which is a pity, but they've done it for a reason.
If there was online, you could keep re-playing levels to get 100% by hoping that who ever you were matched with had the right Skylander for the right door to open. Since you only get 3 to start with, but there's 8 different doors, you'd need to go out and buy at least 5 more unique types of Skylander.
I completed the game yesterday and as fun as it is, i'd love to 100% it, but i'm not going to buy 5 more toys, at £7.99 each just to do that. Online would make sense for people that this game ISN'T aimed at, but from the game devs point of view, they want you out buying the toys.