They've gained a couple of levels since I last played Skylanders..
Dark Spyro and Chop-Chop are both Lv8, with Stealth Elf not too far behind. Again, Eruptor is the lowest leveled character since most of the elemental-oriented challenges focused on Air, Water, Life, Undead, Magic, Tech, and Earth so far. Only at least twice I had to use him for the fire elemental challenge and the fire elemental challenge.
Dark on the other hand, due to the daily elemental EXP boosts was on magic and water, I took advantage of that by putting him on the second slot for the remainder of the playthrough, while Chop-Chop did most of the work. Since this is DARK Spyro, my guess that most of his combat was a hit-and-run stradegy, like most randged characters.
I seem to be more direct though, so Stealth Elf, Eruptor, and Chop-Chop were suited until the Hecktor challenges, which require you to speedrun a level within a certain time limit, with that you need a faster character, oh well. At least there are two chapters 100% done in two levels.