I have done 1 more testing her against ElizaBeth’s Fiends.
If your heroes have Elizabeth’s Fiends and you fire Hulda, then the -24% undispellable mana generation decrease status ailment is overriden with the Minion’s undispellable +24% mana generation increase buff.
The Fiends are disappearing in about 3 rounds as the minions will heal enough to remove them in 3 turns.
If your heroes have Hulda’s minion’s +24% undispellable mana generation increase buff, and later the opponent’s Elizabeth’s fire, then the Fiend’s -24% undispellable mana generation decrease status ailment is overrides this buff.
But you just need still need to wait 3 turns if your minions are still allive to remove the Fiend by Minion healing and the ailment too.