In the second wall to wall pull it is important to use stuns on the big colossus cause he has a aoe swipe that deals massive damage especially when dds stand in front of the boss.

Since you play whitemage it seems, what you can do if your tank has problems with mitigating himself is use your aoe hot heal, your hot zone, your normal regen hot and then use holy once with swiftcast. Keep the tank as your target and cast holy from time to time cause stunned enemies are basically mitigated damage. Holy is a very important spell for whitemage when it comes to making wall to wall pulls easier, easily the most overpowered spell for big mob groups in the game.

Much of it depends on your tank tho, if they don't know how to mitigate properly you can heal as much as you want it won't really matter.