your DT strategy doesn't work at all !!
I was in leage 16 and dropped all the way to leage 21 again only to find stronger and stronger opponents in Dark tower. Also decks that would have easely won in leage 19 now loose in 21... they spit out legendary cards as if they started the game getting them?? Also the HP of the opponents is prety high all the way...
if you ask me, it isn't based on league which opponents you get both in dark tower and in league fights, but rather on your own level, and you can't manipulate that.
So anyone wanting to use this method, just don't...

(fact: while advancing through my leagues I got 4 to 6 dark tower wins, never 7, so I dropped in leagues to get the 7 once. when I was in league 19 I tried, I won 3 fights, then I tried again in league 20, I got 2 win s and totday I tried in league 21, and only managed 1 win, facing opponents with 155HP 3 times while 'only' having 127 myself.... while back when I had 92HP, I managed to win more often).