Turned out that I've been worrying too much. I swapped barb for a priest, didn't bother with wizard. Did all bounties as you guys suggested, finished WM2 and some side quest. Ended up dinging 16 right before pulling Thaos. I think it would be MUCH harder on lvl 12 (or if I agreed for content scaling), but on 15-16 both Kraken and Thaos ended up being zerged without any clever tactics (2nd rogue died on Thaos tho).
Thank you all guys for the tips!