Lately I've been assigned the role of protecting the back line of the party while another tank handles the front line players. I find the SM is quite adept at this role, especially in offensive sovereign gear. The SM is especially capable of frustrating melee trains to no end.
The SM has a few tools to help him keep the enemy melee line from reaching his friendly back line. First, the SM has access to a zero cooldown snare and a zero cooldown single target knockback with a tactic. This will effectively take a single target at a time out of the fight every 2-3 GCDs if you use Sudden Shift to skip normal balance. This is best used after a melee DPS pops some sort of cooldown to make him waste it. I've been against Forceful Shock in the past, but it's quite useful for this purpose. It will see more use in my slots.
Next, the SM can reduce an entire melee train to useless at morale 2. Wings of Heaven into the group and then pop Gusting Wind to send the entire enemy train away from your back line. Wings of Heaven is a morale snare meaning it's unbreakable and at a duration of 10s, it will be awhile before they can reach the back line again. This will usually buy your back line 15s of safety. If Wings of Heaven is on cooldown, you can also pop Leading the Charge! for your back line to let them get more distance. Using both Wings of Heaven and Leading the Charge! at the same time is overkill in my experience and leaves you more at risk of not having a cooldown available to save your group when it's needed.
In the worst case scenario, which is the enemy has Immovable and you don't have Leading the Charge! up, you can reduce a melee train down a third of it's normal damage against a target by using both Guard and Challenge just like any other tank. You also want to use your debuffs against the enemy's best melee DPS to further reduce incoming damage.
I know some of you are thinking that it would be easier to just use a Knight and stagger. You would be right in a sense that it requires fewer button presses. However, the stagger works off Unstoppable and the SM punts work off Immovable. You could almost chain CC a melee train that bunches up with both a Knight and SM working together making it easy for a ranged group to pick them off one by one.