When the pull turns
Hamstring the extra mob, peel away, and finish the first target.When the health bar drops and the pull stays open, swap to a shield before beginning the retreat.
Retaliation and 
Shield Wall share a long cooldown and solve different problems. Stay in front of melee attackers and fight while Retaliation is active. Use Shield Wall when you need to survive the retreat.
Dungeon rules
Bring a current weapon before tanking so normal pulls stay short.
Name the Polymorph, Sap, or trap target before the pull.
If the healer is underleveled for the instance, leave the group.
When the healer hits half mana, stop before the next pull.
Leave now
Leave when a runner heads for the next camp.
Leave if Shield Wall is active and HP is still dropping hard.
Leave when an add lands and no cooldown is ready to peel it.