Having trouble getting those 75 mobs scared, here's an easy way to do it. You can also use this method to unlock other achievements that need for you to gather a lot of mobs.
Tips and Tricks
First, you need a lot of Health and Levels.
I suggest you be around Level 35 or 40 and having more than 50 Health.
Next, get the Heal Spell and both the Mana and Health Medallions and the Fear Spell of course. You need all of these Level 2 but Fear at Level 3 so the fear propagates.
You need your shield at level 3 just in case, if you have played this game so far, you already have this though.
Having a Fan will help the skeletons make other skeletons fearful as they run away.
Remove things that can kill mobs in automode, like Claws and Splitting Arrow , you want mobs alive.
An artifact with Better chance to Dodge skeletons will greatly help but its not necessary.
Dont worry, this is easier than you think.
Now its time to fight the Antipope, you need to hit him enough for him to summon a lot of skeletons, use the doors to get out and move to another area of the battlefield. Do this a couple of times.
Eventually, the antipope will summon mages that teleport mobs to your location, make sure to not kill them, as they are key to bring a lot of mobs to you.
This is where it gets fun as you are surrounded by about 40 skeletons trying to kill you :3