I will do what I can!

So Limbo has 4 abilities - Banish, Riftwalk, Rift Surge, and Cataclysm. He's definately a caster frame and not very tanky, but you can use his powers to control who is capable of doing damage to you so you can deal with the squishyness. There's more than one way to play him, but since you like defense, I will explain how I use Limbo just for defense.

Before explaining, let's go over the Rift - http://warframe.wikia.com/wiki/Rift_Plane All of his abilities revolve around this alternative plane of existence, the Rift. Your first ability sends enemies to it (dealing damage on entry) and casting it again will take them out of it (again dealing more damage). You can also send teammates there (no damage) with this ability. The second ability sends you there. We'll get to the others later. The concept here is on the rift plane, your weapons and enemy weapons/melee can only connect with targets on the same plane. So if you are in the Rift, and your enemies are not - they cannot hurt you. Some things will cut through (poison I think) but for the most part you are safe. This works vice-versa as well, you can stay out of the rift and banish strong targets you don't want to deal with until later.

Here's the kicker - you and your friend's warframe powers work in the Rift vs all targets. Their Rifted/non-Rifted status has no bearing on Warframe powers. Also being in the Rift increases your energy similar to (and stacks with) the Aura energy siphon.

His 4th ability is the most important if you are the defensive frame on the pod (i.e your squad doesn't have a Frost or other alternative) . His 4th ability puts down a bubble where all who are inside are in the Rift - including you and your friends. So if they are outside trying to shoot in, they won't hit, but you also can't hit them. You can however use your powers. So you bubble the cryopod to force all enemies to be in melee range to attack the pod. This is where his 3rd Ability comes in - Rift Surge. This ability multiplies damage you deal to enemies in the Rift when you and your allies are in the Rift. With a power strength build, this multiplier gets absurdly high.

So - stay safe in the Rift, be good at melee or bring a friend who is, bubble the pod and stay inside, and keep re-casting Rift-Surge and everything will die by the blade.

High duration and power strength is a must. Only Cataclysm is adversely affected by range loss, but I have a pretty small cataclysm with the mods I use but the pod is still safe. In fact, a smaller Cataclysm only means you engage even fewer enemies in melee while defending. Also since you and your friends will stay centered, don't be afraid to use the lasers in the TXD tileset, as you won't be running around to get hit by it.

Oh and you cna't pick up items in the Rift, but you also cant get killed by those damn lasers and are great for reviving hallway heroes.