As a mage for the horde, I think Aethas Sunreaver is the strongest mage they have. But I don't think he's as powerful as Jaina.
Theres also Meryl Feltorm (becomes a Mage Follower). He was one of the original member of the Council of Tarisfal, he was the guardian of Med'an and have about 3,000 years of existence and knowledge. He's undead but I don't think he's really in the Horde, but let's pretend he is.
The only one I know could kill her is Garona, as she almost killed Kadgar in WoD. (But she's not a mage)
The Horde is more melee oriented overall. Power-wise I think the Alliance have stronger heroes. Even if Hellscream/Garrosh (IMO the strongest Warriors in the WoW story) could wipe an army by themselves, they can't destroy a building or wipe a city as fast as a mage could.
Vol'jin is supposed to be very powerful as a Shadow Hunter, but we barely saw an of his skills in the matter, making him a only a Hunter (bowman) who can talks with the spirits so far.