If your on easy, then just use regulate until he runs out of focus and kills himself.
If your on medium, consider using some debuffs from the cold side, even if youa re a hot hydro.
If your on Heroic, you need to be a cold hydro, period. You will need ice wall to defend/heal your teammates from tick tock and isolation. FLash freeze will buy you time when neccesary against the baron. The rest is up to you, but I picked mind freeze and glacier as my other two debuffs on the cold side since they more impair your opponent rather than make him weaker to your attacks. Glacier will make the baron do less damage to you and your teammates, and then mind freeze will limit his array of attacks to use against you. Keep using regulate, use ice wall when a teammate/you is weak and needs healing from tick-tock/isolation,a nd flash freeze when neccesary. When he goes into holy scars mode, hit him with your other debuffs. Also, don't forget to respec your ability points into vitality, you need to survive the long haul of a 95k health opponent.
Well, either way, baron is a long ass fight as hydro, since regulate takes away 10 focus at best from him.
However, in heroic mode, it is NECCESARY to use some of the cold hydro attacks like mind freeze and flash freeze as well as ice wall. It becomes impossible to hold off tick-tock if you only have hot hydro skills.
Forget about cutting down his focus; he will do it himself, so just focus on keep your team alive with Ice Wall and Glacier to soften his blows. I do NOT recommend using Shatter. Maybe if he has "holy scars", but getting pounded on with nearly 50% more damage is not worth doing 2k damage. Keep yourself alive.
Then again, if you're not on Heroic, it's pretty hard to lose. Just fight him as you would any other boss and keep an eye on the health bars.
