I personally train them all to 10 as if they were fodder, then put them together, give them an elim, the might fragments and the first ambush, then go on from the Delta.

I find it takes way too long to level them individually after 10, but then again I can accept doing levels 10-15 the regular way if they have their stones. The ones I really can't do are those before they get elim, that's way too slow and frustrating. If it's for the drops, I'd rather spend half an hour with 25s in the woodland path than have to grind it with baby pets.