Yea, you're well over 1500 combat regen below where you need to be for this fight. That definitely is a lot.
To help, Id completely reforge out of Haste if I were you. Mastery and Crit trump it for this fight. The mod
reforgelite is great if you're not using it. Install the mod, then go talk to the reforge NPC. You should get a window that pops up. From the drop down click on Priest->Holy and then manually set the secondary stat weights. Set Spirit to 200, Mastery to 160, Crit to 140, Haste to 120, and Hit to 100 and click on calculate a couple times. Once you do that, click show and a new window should pop up with some gear check boxed. Accept the reforge changes it suggests, and it'll do it all for you. If you need to go back to shadow, you can preset it to what you need to make it much easier.
Aside from that, you may have to talk to your Resto druid and ask them to give you their Innervates for this fight.
Put Lightwell in a good spot, and tell anyone who has Searing Plasma to click on it.
Good luck!
We just started doing this guy this week finally. A two night a week schedule really blows for progression. :/