Skittering Stone I/II could be cool with a small adjustment.

Remove the kill requirement and change it so earthen structures being shattered trigger the summon.

In its present state, it doesn't work well for bosses.

You can try a temp minion build with living lightning + skittering stone.. but then your allies will instantly pop the shield wall. No amount of HP on it will make it functional in that setup.

Due to how oppressive the condition for triggering it is, you can barely play with it.

Living Lightning and the change to Unearth are good examples of keeping temp minions fun/useable in various situations. This one would be really fun to add to any build, since it also scales %dmg with STR, but it just has a very oppressive start condition making it a nearly dead supp gem imo.