Having only one melee character significantly reduces the total area you can cover with attacks of opportunity, and you are incapable of arranging for double cover of anything. The first means that enemies will usually not have to spend many AP on getting around your AOO zone, the second means that even if they do choose to move through it, it is only going to be moderately painful.
The way to make the enemy focus on your front line is to make not doing so appear more costly to the enemy, and this can be accomplished through AOO, AP costing hazards, or damage causing hazards.
Well, that and take the Stench talent on characters you really want to dissuade the enemy from engaging in melee.
And if you don't want that, then you'll just have to work with ranged that are roughly as durable as melee characters against physical damage, which is thankfully trivial to accomplish by using Become as Air.
As for tanking, forget it. That's an MMO teamplay mechanic that has little place in role playing games.
