It takes practice, but i assure you with 500 mind, it's more than possible.

Don't regen anything until the tank has pulled into position. Holy spam a few times, divine seal cure II like crazy. Something i like to do sometimes is holy until the tank is almost dead then benediction and holy some more.

Ultimately, a dungeon goes as smoothly as a healer can heal, despite tanks thinking they can rush through everything. If it comes to it, there's no shame in just asking for half pulls while you get the hang of it. Speed runs aren't for everyone.