I don’t think Spirit in itself is a badly designed runeword, but it’s more a reflection of the lack of other good caster shields available in the game. But, it is cheap, and if it required a high rune or two to make, then I think that will be well and truly balanced.
The only reason why Spirit was made using cheap runes is because it was originally a Ladder-only runeword and can be made in swords, so it helped out in game progression early in the ladder season. As Spirit is no longer a Ladder-only runeword, it currently stands as being overpowered for the runes required.
Edit: I’ve made a couple of suggestions to change Spirit here:
The current runeword in question though creates an item that is far better than any other option out there for physical based fire builds or even non-sorc based fire caster builds such as an Armageddon Druid, and is even a huge buff to the Necro’s corpse explosion.
As much as fire builds can still use a buff, this should be addressed by making more targeted skill balance changes rather than making overpowered runewords, as this will result in “all fire builds MUST use this runeword to be viable” situation as well as potentially buffing things by accident that don’t need to be buffed.
Making overpowered runewords are also much harder to correct later on as it’s less damaging and awkward to do skill balance changes than to delete runewords that have come into existence, and have already been made by everyone, but then are no longer able to be made because of a correction.