[Blade Tempest]ωϖ—Rapidly twirling blades deal 70% of weapon damage to enemies in front of the attacker every 0 sec for 4 sec.
[Repelling Wave]ωϖ—Knocks nearby enemies back, inflicting 4163 to 4837 Fire damage, and stunning them for 3 seconds. Used while summoning a clone.
[Summon Clone]ωϖ—Summons a clone with the same health. Used at 50%.
[Enervating Brand]ωϖ—Brands an enemy target, siphoning power from the target and any allies within 12 yards of the target. This effect occurs every 2 sec, and reduces affected enemies' damage by 2% per application, while increasing the caster's damage accordingly.
[Siphoned Might]ωϖ—Increases the caster's damage by 2% per applied brands.
Strategy
This fight needs only 2 tanks for 10 player, 3 for 25 player. The trick is to make sure that when any raid member is affected by the debuff, they move away from the rest of the battle so as to be at least 12 yards away from any other raid member. It is especially important to have all melee back 13 yards away from the Main Tank before cleansing the debuff so as to ensure that they are not affected either. At the very least, the Tanks CANNOT be debuffed when Baltharus initiates his whirlwind attack.
At 50% health in 10 player (67% and 33% in 25 player (needs confirmation)), Baltharus will clone himself. The offtank(s) has to taunt the clone and keep that with him. Now each tank has a copy of Baltharus. The DPS must attack only the original one.
Be careful around his knockback. It is possible that he might knock you back into the group behind him containing a Charscale Commander. If the commander gets off Rallying Cry, it is likely that the raid will be quickly demolished.
If this seems to be a problem, pull him back some to the north, closer to the entrance portal.