This fight is very easy to win without even any effort.
Alguin has a power, called "Alguin' Gambit"
the power takes 6 turn to charge. Once charged it will kill mangar in a single blow.
All you have to do is keep Alguin alive.
The good news is Alguin has high HP and can live for 6 turns without even you trying to save him.
I should add that I played in Normal difficulty. In hard mode this might not be possible.

In the first turn click on Alguin's Gambit power and then wait for 6 turns,

Bard's Tale 4 Game Guide: [ Ссылка ]

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