"Mighty Guard" is one of Strago's lores, which in theory can be learned from Earthguard. In practice, Earthguard appear with 1HP and automatically dies almost immediately. Even if Relm was fast enough to control them, they don't have enough MP to cast Mighty Guard.
The way to learn it is to have Gau learn it from Earthguards first, and then have Strago learn from Gau. It took longer than expected but I have finally done it, and now I have completed all of Strago's lores. Only later did I find out that there are two other enemies that also uses Mighty Guard.
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