Garrosh would own Arthas. Arthas as the Lich King though now that would be a different story, Garrosh wouldn't stand a chance.
Meanwhile, Arthas was trained and bested some of the greatest fighters the Alliance has ever had. Muradin, Uther, various other paladins. He sparred with Varian occasionally (the two were pretty even), whom has beaten Garrosh on numerous occassions.
Reg. Arthas V.S. Reg. Garrosh; Arthas wins.
Reg. Arthas V.S. Empowered Garrosh; Garrosh wins.
D.K./L.K. Arthas V.S. Reg./Empowered Garrosh; Arthas wins.
Arthas is too good of a fighter, and his abilities granted to him as both a regular death knight and as the Lich King are too good for either form of Garrosh to beat, imo. It'd be harder to judge if Garrosh was able to live up to his family name as a fighter, but, well, he didn't. He was slightly better than average at best, and even that is likely because he was unusually big for an orc.
we beat garrosh without external help, we only beat arthas due to ashbringer plot armor.
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Fully empowered Garrosh vs Arthas (paladin) and Arthas (Death Knight): Garrosh wins.
Garrosh (TBC) vs Arthas (paladin): I'd say Arthas
Garrosh (Cata) vs Arthas (paladin): Garrosh
Garrosh (Cata) vs Arthas (DK): Imo Arthas again
Fully empowered Garrosh (with Kor'kron) vs Lich King (with some ghouls, yminar and few frost wyrms and such): Lich King
Why? I find Lich King's abilities far stronger then Garroshes mostly because he can rip souls from his enemies and general advantage against living. Garrosh has a big axe, shitload of interesting abilities and raw physical power... Also Lich King never gave us his full potential. It was all a "game" for Lich King, Garrosh actually wanted to pain his throne room with our blood. Lich King was laughing.
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Garrosh couldn't even beat Cairne in a fair fight. Arthas was hunting and making dreadlords run in fear long before he became the lich king. Garrosh beat Taran Zhu but not effortlessly. I'd say Arthas would win every time.
I'm a Forsaken. I kill firstborns while their mamas watch. I turn cities into salt. I even, when I feel like it, rip the souls from little girls, and from now till kingdom come, the only thing you can count on in your existence is never understanding why.
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