BK is always first. Do not waste a single DE on training DE troops or researches until you could level up BK to it maxed level. (lvl5 as TH7 and lvl10 as TH8)
When I enter to TH7 45 days ago there was no DE drill. I build the DE storage and upgrade BK to level5 non-stop in one week.
After that I did the researches for minions lvl2 and hogs lvl2.

