Once you’re done with Future, duck around the buried castle bit to the North and you can grab a Nugget, then follow the North coast back to the East for a Heart Scale.  With everything and everyone now grabbed and beaten (respectively), return to the ruin entrance and bop on down the stairs.  Incidentally, your team should be around level 22-24 right now if you have four or five team members- slightly higher if you have less, slightly lower if you have six.

This brings you into Relic Castle.  In here there are still pokemon- just fewer of them.  Sandile, Yamask, and Sandshrew are all found in here, around level 18-21.

Head North and talk to a scientist to find out about ‘quicksand’, which is what the odd square pits in the floor are.  Bypass the first pit and carefully tread the corner of the second, and you can find a Psychic who wants to battle.. Because of his noisy Sigilyph.

Battle: Psychic Perry (736 Poke)
Opponent: Sigilyph Lv 23

Sigilyph is a dangerous opponent at this level, so take it down hard and fast with the best attacks you have- physical and Dark if possible.  Electric or Ice can work too, but those are less likely to be physical attacks.

Once you’re past the psychic, head downstairs to the first floor basement.  You’ll immediately encounter.... yet another psychic!

Battle: Psychic Dua (736 Poke)
Opponent: Gothita Lv 23

Walk past her to pick up the TM for Rock Tomb, then return upstairs.  This time, intentionally fall down the left quicksand pit.  This will deposit you in a room downstairs with a third Psychic in it.

Battle: Psychic Low (736 Poke)
Opponent: Solosis Lv 23