If you use vesta's spear special on monsters, they will not be damaged. You will still get xp for missing hits.

If you 1-hit with the spear's special, a visual bug occurs (still shows full healthbar) and the monster dies. However you will be hitting nothing (you still will get xp drops), until the dead monster respawns.

Also tried this in safe wilderness with Ca422 and it didn't affect Pking.
Thanks to Ca422 for reporting and helping me out with it.