Even if he hits a low armor target he is still almost guaranteed to take out at least one troop and keep the turn, and that is for 14 mana, it is really quite ridiculous.
For all intents and purposes his spell right now is probably stronger than the one that is coming for the Great Maw which is restricted to one use per battle.
If you’d change Bone dragons text to "destroy the opponents first troop, deal 3-30 damage to the second one, gain an extra turn" it would sound outrageously overpowered, but in actual gameplay its spell results in exactly that at least 80% of the time(and mildly less catastrophic the remaining 20%).
Sheggras spell is also quite close to that ridiculousness, and mana cost isn’t even the real issue, you can’t really put an appropriate manacost to spells that powerful.
Compare it to spells like Jarls which also cost 16(also red/brown exact same cost as Sheggras), spells like those get dwarfed in power by the skull dragons.