Do you like Future Sight bugs, Jan? I do!
Future Sight is using the special attack of the Pokemon that is currently in the battle, rather than that of the user (and consequently is most likely not receiving STAB if used by a Psychic-type which is later switched into something of another type).
At least it isn't completely broken.
There are 7 to find: one on route 9, two in GDC, one in nectar woods, one on route 7, one in oblitus town, and one in the oricorio field in darchlight woods.
+2 252+ SpA Smeargle Judgment vs. 0 HP / 0 SpD Hariyama: 76-90 (50 - 59.2%) -- guaranteed 2HKO
0+ Atk Hariyama Force Palm vs. 252 HP / 4 Def Smeargle: 126-150 (111.5 - 132.7%) -- guaranteed OHKO
very easily (levels were set to 34 for both)
EDIT: right, field effects exist.
+2 252+ SpA Smeargle Judgment vs. 0 HP / 0 SpD Hariyama: 169-201 (111.1 - 132.2%) -- guaranteed OHKO
still, if it's modest you can outspeed, and rock and steel-types can take a hit since Sacred Fire is Physical.
+2 252+ SpA Smeargle Judgment vs. 0 HP / 4 SpD Wormadam-Trash: 59-70 (62.1 - 73.6%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Smeargle Sacred Fire vs. 0 HP / 0+ Def Wormadam-Trash: 40-48 (42.1 - 50.5%) -- guaranteed 3HKO after Leftovers recovery
You can also get Cell Batteries from wild Charjabug, and Metal Powder from wild Ditto, the latter of which there is only one. You can keep running from the ditto until it has it though.