I don't think any of the Clayton versions of the 5 cores are offered in the summer unfortunately, so you'll most likely have to overload or take an extra semester.
ETC1000 isn't too bad. I actually quite liked it. There is a little bit of maths, but you can mostly gloss over it, and don't have to go very in depth. Most of the unit is interpretation from what I recall, with the odd calculation on the side. The notes are very comprehensive, and you get heaps of past exams. The lab classes for it can be pretty fun if you have a good tute. Brett's lectures are pretty entertaining too. You get given a CD with pre-recorded video lectures, so in the main lectures he just tells stories and goes through examples.
Well normally, people say that business law is the hardest of the core commerce units, but you don't have to do that one. If I had to rank them from "hardest" to "easiest", (bare in mind I haven't done management yet)
ACC1000
ETC1000
ECC1000
MKC1200 (just rote learning a bunch of definitions)
MGC1010 (open book exam)
I'd avoid doing marketing and management in the same semester though. Not because it'll be hard, just really boring.
But honestly, don't worry too much about the difficulty. It is so subjective, and they are all of similar difficulty. Also, they won't seem hard or time consuming at all compared to your law units.
If I were you though, I'd do accounting and micro in first semester, then stats and either marketing, or ACC1030 (the second accounting unit). You can change your second semester units after first semester though, so just put something in now, and change it later.