[Faceman]

with the right party and a bit of luck you can kill him at any level
tactical spoilers
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All you need is to lower his resistances and/or saving throws - then use the one-hit-kill method: i.e. cast finger of death or harm or quivering palm just one of these one hit kills. Harm is particularly effective because it does not care for saving throws or resistances IIRC, yet it requires you to hit fiirkraag on your very first swing with it (and once afterwards anyhow but that shouldn't be the problem).