That one is a classic.
Fortunately the game is never over in Bard's Tale.

Try casting SPBI on that creature (needs a free party slot); gets you a monster that frequently breathes with a breath that is more powerful than that of a Red Dragon and more powerful than anything you could put out with a spell or item in BT1.

(By the way, in BT2 there are breath attacks that outdamage Mangar's Mallet.)

dont have a Wizard.

Get one, and level them up until they get 5th level spells (IIRC). I find this particular spell incredibly fun to use, especially in BT2 and BT3 where you can drop special party members off at the guild and re-recruit them later (can you do this in the BT1 remaster?)

Alternatively, that Spectre Snare that you can get at the very end of the game casts this spell when used as an item, but if you still don't have a wizard, you are probably not ready to get it. (The last couple floors of Mangar's Tower are probably the most dangerous floors, combat-wise, in the entire series.)