Today, a friend and I (blood dk + retridin) tried to do the I've had worse achievement. Neither of us got hit by ghoul explosion visually, nor according to the combat log, but the dk's blood worms did get hit. We didn't get achievement.

Now the achievement says:
Defeat the Black Knight in The Trial of the Champion on Heroic Difficulty without any player in the group being hit by a ghoul explosion.

As far as I know blood worms aren't players.