Is it completely random what i should trough down the well, or are there some special items that are better for this?

No, there is no item which is better than others. I suppose that the best items would be the ones worth the most if it comes out to be good.

But its all random so you might as well just well everything you've got

well there is one think you can keep in mind
those monsters will only have 1 charm
so if you throw a heal down, they won't attack you with a charm, only a weapon
if you throw in an attack charm, they won't heal themselves

pretty sure that's how it works

cool gr8, thx for answer!

This Simpel and Easy

From what I remember reading they will use the item that will drop when you kill them.