the "hurt by undefined" is a recurring bug.
when entering an affected area, anything that is already present (wild pokémon, other trainers, your and their pokémon) will be invisible to you. anything that appears or enters while you are still in the area will be visible. if i came to -3,2 and you were there next to the exit, you'd be able to see me, but i wouldn't be able to see you, because you were already there when i arrived.
the "undefined" is the invisible wild pokémon attacking - they are still there to be fought, and can be fainted as normal, but care will need to be taken so as not to faint yourself on wild pokémon you cannot see.
note that anything that does spawn while you are there will become invisible again if you leave the area and return later.
should you need to use that area for training, there are three other interconnected graveyard tiles with the same spawn list. it's a good spot for training HP EVs.