Here are some tips to help as a WHM:

Regen + Benison is an excellent combo for covering general damage the tank takes, so make sure to keep these up during a pull.

Holy is a GREAT mitigation tool. The stun you get will give 4-7sec of zero damage when used back to back.

Afflatus skills should be your Go-To first use abilities when you have them. Combo them both with Plenary Indulgence to get an added 200 potency. Also note that PI is a buff and doesn't dissapear after it's first activation.

Assylum, Assize and Temperance all mesh extremely well together. I tend to alternate pulls when using Assylum and Temperance given their C/Ds.

Tetragrammaton is excellent to use when you do not have an Afflatus skill available.

Benediction should not be kept as a panic button. It is the most potent, single target heal in the game. Plan your use of it. Doing so lets you save other resources for future pulls.