Spear Cretin ???

[Toymanrtk]

To use Spear Cretin's effect does my opponent need to have a monster in the graveyard or just me?

 

[John Danker]

To use Spear Cretin's effect does my opponent need to have a monster in the graveyard or just me?

It's like Gilasurus, you get the effect even if your opponent can't take advantage of their part of it.

So, in answer to your question, your opponent does not need to an elligable monster in their graveyard in order for you to benefit from Spear Cretin's effect.