Also, something a lot of guides don't put in but I should warn you about (speaking from experience):

When Ifrit uses Hellfire, assuming you survive, he will start to walk back to you (assuming you still have enmity.) If you're still standing in the spot you were originally tanking him, his pathing will be messed up and he'll move to your left, causing positioning to be off.

When Ifrit jumps to use Hellfire, after you pop sentinel, move a little bit to your right. This will correct his pathing and when he approaches you, he'll move back into proper position. Then all you need to do is move directly in front of him again and continue tanking.