Despite having only very little time last 3 days (around 2-3 hours a day) I managed to get 11 skins on this Mage Tower.
Before I got: all Xylem, 3/5 Sigrin (Ret, Ench, Assa) and 1 Feltotem (BM Hunter). Now I have all Xylem, all Agatha, all Feltotem, 3/5 Sigrin, 1/5 Twins (MM Hunter) and 1/6 Kruul (Prot Warrior).
All chars 920-940, mostly shitty legendaries (nowadays I farm Lock/Druid for specific legos + something for my Arcane Mage that would give him some survivability). I was surprised how easy Agatha is with this gear, only Outlaw took more than 1-3 tries.
So it is very doable even if you start so late into expansion. It's just frustrating than you must schedule your free time around this stupid building mechanic.
EDIT: Assuming that Pre-patch will hit 3./4. July (6 weeks before expansion - just like Legion, although I hope they launch it 10./11. due to USA holiday) we have 4 MT left on EU (3 if we're unlucky).