This is going to be an avatar version of Beowulf as you can tell by the title so here is the cast.
Beowulf-Zuko
Grendel's mother-Azula
Hrothgar-Iroh
Unferth-Long Feng
Weathlow-Katara
Hild-Suki
Estrith-Yue
Cille-Mai
Garmund-Aang
Eofor-Sokka
Wulfgar-Piando
Yrsa-Li
Aesher-Teo
Cain-Sozin
Grendel-Hei Bei
Wiglaf-Zhao
Hondshew-Shinu
Olaf-Hakoda
Aethelbeorg-Lo
Ursula-Toph