Ah so your problem is not the invincibility of a quinted spirit nymph but rather that you think the card achieves the same result cheaper than the current alternative. Your solution was to increase its threat level to increase its mortality by a small amount.
I think your efficiency concerns are unfounded.
Stoneskin requires only 12
|6
(and +5 draws and a temporary stockpile of 67
which can be spent later for shriekers)
Spirit Nymph requires 109
|109
(and +0 draws and 800|600hp of healing meaning it requires many miracles)
109
+800hp of healing vs 12
+5 draws and a stockpile of only 67
.
I think Spirit Nymph might have too high of an activation cost. But what do you think?