After dealing with those stupid boulders I finally reached Tara the Psyquic Gym leader.
What can I say? Dark types >>> Psyquic types, Krookodile destroyed the gym (and it got way stronger than i intended since the gym is around level 42 while Krook was hitting level 50 for some reason)
We can now use Waterfall (but I don't have the HM...)
There's a Team DH building here so maybe it's there